Why does Christianity have Satan as well as God? Just as a little background:

I study religions as a hobby but I don't really worship them, although they do dramatically affect how I perceive the world. I like to read scriptures. I have read the 5 vedas, Mahabharata, Tuttvartha Sutra, parts of the Tripitaka, The Dhammapada, The Egyptian Amduat, The 12 Gates, The Zend Avista etc etc. Religions interest me, I find them absolutely fascinating to read, and to reflect on. I don't see scriptures as being "right" or "wrong" or "true" or "false" for me they seek to encapsulate a certain amount of wisdom that we can all learn and benefit from so for me, i have no personal conflict between religions, nor do i get offended by strong religious opinions, nor do i feel prolonged loyalty to any particular religion or scripture; although I would be lying if i said I didn't have favourites!

Currently, I am taking a break from Egyptology to try and understand the Bible, which I did partially read as a child as I was brought up Roman Catholic, but I didn't very well understand it, but I'd like to, and there are things that confuse me.

One thing that I am particularly struggling with is that in Christianity, we have the concepts of God as well as Satan. This for me is tricky for me to get my head around, because in many other religions - particularly polytheistic religions God(s) are not necessarily good, nor bad they can be either, or neither depending entirely on perspective. For example: the Hindu God Shiva is depicted as a destroyer, but also as a creative force that clears away the old to make way for the new. Or, if you look at Jainism, "God" is pantheist and above reproach because "The absolute is impersonal", so in that context, it's irrelevant to question whether God can be good or bad, since the Jains see the construct of God as literally being the world around us - the very laws of nature.

The question:

So what I am trying to understand is, as Christianity is monotheistic, why Satan features in the bible at all. There must be a reason for it, and I've no doubt it's a good reason. And my concern is that by not understanding this reason, something fundamental to Christianity is missing from my understanding.


Did the Old Testament always have a concept of Satan or was it added
later?   
If so, why was it added? (For example some of the scriptures in the Bible that were present in Coptic scrolls in the Nag Hamadi library e.g. the "Exegesis of the Soul" were destroyed due to a clearout of suspected heretical material in the Bible that may have crept in over the centuries and so do not feature in the 1611 KJV many centuries later.)
Was it added because devout people were afraid to depict actions of God as being negative and they were scared of being seen to blaspheme?  
If Satan is there to carry the burden of all evil within the world, are actions such as the anger of God in the old testiment, good or bad, or neither?
Is there any clue in the Bible that gives indication as to why God's anger is not the responsibility of Satan?
Is there an official stance on this, and is it based on the scriptures, or by a figurehead, such as say a saint or a pope?


Essentially the thing I am confused about is the partition or the line between the actions of God and the actions of Satan. Can God do evil, or is it only Satan?

Apologies, I am really trying to make these questions as historically based or scripturally based as possible, and I trying as much as possible to avoid asking an opinion based question, but in religions there is an awful lot of opinion in interpreting scriptures and the history of such things can be clouded over long periods of time by wars, cultural changes and the loss of material.
First off, I like your question because it made me think. Second, no answer can be given that does not also make as many assumptions as your questions. This answer assumes Biblical Inerrancy, a traditional timeline for the writing of all the books, and the assumption that not much editing has occurred between when the books were originally written and today.

1. Did the Old Testament always have a concept of Satan or was it added later?

From a literary standpoint, themes related to Satan run from beginning to end. The only addition is of depth, scope and detail. The first promise of a savior is in Genesis 3:14-15, and that is spoken directly to the serpent. The last reference to Satan is in Revelation, the last book of the Bible.

One place where you can argue that information about Satan was added later is in Job. The literary structure implies that Job is unaware of Satan’s involvement in his suffering, so that detail must have been added later. Tradition holds that Moses wrote down the Book of Job a few centuries after its event occurred. Another view is that Joshua, Moses’ scribe, who finished writing the Pentateuch after the death of Moses, wrote down Job. There are a few phrases found only in Job and Joshua, supporting this view.

Job is interesting because one of his first statements in his complaint is:


  May those who curse days curse that day, those who are ready to rouse
  Leviathan. (Job 3:8, NIV)


God describes Behemoth (a terrestrial monster) and Leviathan (an aquatic one) in detail in Job 41. Then in Revelation 13, two beasts, one from the land and one from the sea, are described. Commentators mostly agree that those two beasts are Behemoth and Leviathan, and the latter is Satan. This makes the final chapter of the first book of the Bible to be written devoted substantially to a representation of Satan.

You might ask, why? Satan, the devil, Lucifer - serves several roles. He is adversary, accuser, liar and murderer. The Old Testament is preoccupied with God as a lawgiver. In a court of law you have the accused and the accuser, testimony and witnesses, criminal, victim and crime. Satan serves as the focus of lying to make false accusations in contradiction to God (especially Jesus) who is the truth. Satan falsely accuses and God forgives. Satan murders and God gives eternal life. Thus, from a literary standpoint, the presence of Satan makes sense. The worldview of both Old and New Testaments is that all things have logical causes and laws that govern them, hence there must be an origin of evil, lies and death, and a good God will reveal it to his people.

2. If so, why was it added? (For example some of the scriptures in the Bible that were present in Coptic scrolls in the Nag Hamadi library e.g. the "Exegesis of the Soul" were destroyed due to a clearout of suspected heretical material in the Bible that may have crept in over the centuries and so do not feature in the 1611 KJV many centuries later.)

My assumption is that it was not added, except as I described concerning the Book of Job.

3. Was it added because devout people were afraid to depict actions of God as being negative and they were scared of being seen to blaspheme?

The Book of Job and Ecclesiastes are each hard-hitting books that subversively question accepted wisdom and false religious ideas that attempt to give simplistic answers to hard questions. Job especially takes exactly the opposite tack of the one you suggest. After relating a conversation between God and Satan in which Satan is given permission to inflict catastrophe upon Job, God never lays blame on Satan in His answer in chapters 38-42. In Genesis, Eve blamed the serpent for her actions, and Adam blamed his wife. God blames no one for Job’s misfortune. He assumes full responsibility - even if that means bad PR for his religion, and Job accepts that and continues to worship that same God:


  Then Job replied to the LORD: “I know that you can do all things; no
  purpose of yours can be thwarted. You asked, ‘Who is this that
  obscures my plans without knowledge?’ Surely I spoke of things I did
  not understand, things too wonderful for me to know. “You said,
  ‘Listen now, and I will speak; I will question you, and you shall
  answer me.’ My ears had heard of you but now my eyes have seen you.
  Therefore I despise myself and repent in dust and ashes.” (Job 42:1-6,
  NIV)


4. If Satan is there to carry the burden of all evil within the world, are actions such as the anger of God in the old testiment, good or bad, or neither?

The Bible distinguishes between God’s anger and man’s.


  My dear brothers and sisters, take note of this: Everyone should be
  quick to listen, slow to speak and slow to become angry, because human
  anger does not produce the righteousness that God desires. (James
  1:19-20, NIV)


God’s anger is foreseeable, justified and proportionate. 

It is foreseeable, because God gives us His laws so we know what to expect, he does not change them without advance notice, he reveals his future actions via His prophets, and then when the time arrives, He executes His judgment as promised. 


  Surely the Sovereign LORD does nothing without revealing his plan to
  his servants the prophets. (Amos 3:7, NIV)


It is justified, because God only punishes according to the truth, cannot be bribed, and does not show favoritism.  


  There will be trouble and distress for every human being who does
  evil: first for the Jew, then for the Gentile; but glory, honor and
  peace for everyone who does good: first for the Jew, then for the
  Gentile. For God does not show favoritism. (Romans 2:9-11, NIV)


It is proportionate. When God sent foreign armies to punish Israel and they overstepped their bounds and tried to cause more harm than He had decreed, He sent distress upon those nations, too.

Israel was exiled for seventy years. They had failed to faithfully honor him on the Sabbath for 490 years, so one seventh of 490 is seventy. Their exile was measured out precisely.

God’s anger is not the anger of Satan who wants to kill. It is the anger of a father who wants to teach, admonish and straighten the path of His children so they will be perfect and can experience joyful fellowship with Him forever. Motive is important. Thus God’s anger is not evil or neutral, it is good.

5. Is there any clue in the Bible that gives indication as to why God's anger is not the responsibility of Satan?

Job finally accepts that the terrors assailing him are part of God’s plan to purify him:


  But he knows the way that I take; when he has tested me, I will come
  forth as gold. (Job 23:10, NIV)


Paul reiterated this idea:


  And we know that in all things God works for the good of those who
  love him, who have been called according to his purpose. (Romans 8:28,
  NIV)


A vital part of the theology of suffering is that God speaks to us in various ways, and one of those ways is suffering. If you look at all of Job’s speeches, you will find that he is publishing a job description for a savior, laying out all the troubles from which he needed God to rescue him. That job description matches point for point every things that Jesus eventually did. So Job’s suffering taught him what was needed in a Savior. Job recognized that he needed a savior with the power of God (could walk on water: He alone stretches out the heavens
and treads on the waves of the sea. (Job 9:8, NIV)) but also the empathy of a fellow, suffering human, whom he called the “son of man”, a title Jesus would later adopt to describe himself:

My friends scorn me;
   my eye pours out tears to God,
that he would argue the case of a man with God,
   as a son of man does with his neighbor.
 (Job 16:20-21, ESV)


Job basically prophesied the incarnation!

6. Is there an official stance on this, and is it based on the scriptures, or by a figurehead, such as say a saint or a pope?

I have based my answer on Scripture. God’s anger surely has many purposes beyond those I have discussed, but most assuredly is a tool that He employs to strengthen our character and give us hope.


  Not only so, but we also glory in our sufferings, because we know that
  suffering produces perseverance; perseverance, character; and
  character, hope. And hope does not put us to shame, because God’s love
  has been poured out into our hearts through the Holy Spirit, who has
  been given to us. (Romans 5:3-5, NIV)


Satan’s Role. It is one thing to say that Satan was not a later addition, but it certainly was an inclusion. The Bible has an author and that author had a purpose in including information in the Bible concerning Satan. Satan is not mentioned often in the Old Testament, and in the New Testament he appears most often as a symbol in Jesus’ parables. Those few appearances occur at crucial times.

There are only five conversations in the Bible where Satan is present. In one (Zechariah 3), Satan is told to shut up and does not get to speak. In two (Matthew and Luke) we hear the same story of Jesus being tempted in the wilderness by Satan, with a slight variation in the ordering of events, we we will count that as one. That leaves Genesis 3 (the temptation of Eve) and Job 1 &amp; 2.

So we have three principal conversations:


In Eden about 6,000 years ago (or more). 
In Heaven (discussing Job) about 3,700 years ago
In the wilderness near the Jordan about 2,000 years ago


In the first, Satan speaks to the creation (Eve) to attack the goodness of the Father in withholding the Tree of the Knowledge of Good and Evil from her and Adam.

In the second, Satan speaks to the Father about Job, saying that the man will curse God if made to suffer. Since Job’s great prayer is that his words be written down, Satan is attacking the Word of God - Jesus. If Job failed and cursed God, then the Bible’s first book could never be written.

In the third, Satan speaks to Jesus about - what? Immediately prior to entering the wilderness, Jesus was baptized and the Holy Spirit descended upon him. The Holy Spirit is the Advocate (teacher that explains Jesus’ words) and Comforter (who helps us with our emotions). Satan tried to trick Jesus with lies and scare him into getting angels to help him avoid suffering, persecution and death. Satan was trying to get Jesus to lose his connection to the Holy Spirit. Later in his ministry, Jesus told Peter that Satan wanted to sift him as wheat. What was Peter’s big moment? When he delivered the keynote address on the day of Pentecost. Peter announced the arrival of the Holy Spirit to empower people, work miracles of healing and enable truth to spread to the whole world.

Take this all together, and Satan’s main appearances (apart from Revelation, the end) are to attack the Father, then The Son, then The Holy Spirit - to create division within the members of the Trinity and cause God to fall apart. Satan failed, but in this failure caused the clear Triune nature of God to be manifest. 

I am sure that God had other reasons for including stories about Satan than these, but he at least had these reasons.