What is the Biblical basis for considering Satan the author of evil? Satan in Job seems to be characterized as a servant of the Lord and in Isaiah Lucifer is mentioned as a destroyer of cities. It's never really clear why evil exists. In a couple places in the Old Testament it is mentioned that God is all good. So what's the Biblical basis for believing that evil comes from the devil?
While the Old Testament introduces Satan and portrays him as:

Inciting David to sin by taking an unauthorized census (1 Chronicles 21:1)
Accusing Job of only walking uprightly because of the many blessings he had received from God (Job 1) and tormenting him when given the opportunity (Job 2)
Accusing Joshua the High Priest (Zechariah 3:1-2)

It doesn't go into anywhere near the level of detail that the New Testament provides regarding this particular being. One of the things we can see in Matthew's account of the Temptation of Christ in the Wilderness is that Satan is alternatively identified as &quot;the devil&quot; and described as &quot;the tempter&quot;. This identification is also made explicit in the following verses in Revelation:

The great dragon was hurled down—that ancient serpent called the devil, or Satan, who leads the whole world astray. He was hurled to the earth, and his angels with him. - Revelation 12:9 NIV
He seized the dragon, that ancient serpent, who is the devil, or Satan, and bound him for a thousand years. - Revelation 20:2 NIV

&quot;That ancient serpent&quot; is an allusion to the serpent in the Garden of Eden - the original tempter.
Elsewhere, we see references to &quot;the evil one&quot;,&quot;the god of this age&quot; and &quot;...the ruler of the kingdom of the air, the spirit who is now at work in those who are disobedient.&quot; (Ephesians 2:2) which from context all refer to the same being. Given that basis, an additional key passage that is relevant to answering your question is John 8:44 -

You belong to your father, the devil, and you want to carry out your father’s desires. He was a murderer from the beginning, not holding to the truth, for there is no truth in him. When he lies, he speaks his native language, for he is a liar and the father of lies.

So in summary, the Bible - and more specifically the New Testament - asserts that Satan is &quot;the father of lies&quot;, &quot;is now at work in those who are disobedient&quot; and &quot;leads the whole world astray&quot; and by nature &quot;was a murderer from the beginning&quot;; Given this basis, a designation of &quot;author of evil&quot; does seem appropriate for Satan.
Unfortunately your additional questions regarding where and why evil exists go beyond the scope of what can adequately be addressed on this site without resorting to primarily-opinion-based answers.