Apparent discrepancy in Isaiah 66:3-4 In Isaiah chapter 66: 3-4, God had this to say.


  3 But whoever sacrifices a bull 
      is like one who kills a person,
  and whoever offers a lamb
      is like one who breaks a dog’s neck;
  whoever makes a grain offering
      is like one who presents pig’s blood,
  and whoever burns memorial incense 
      is like one who worships an idol.
  They have chosen their own ways, 
      and they delight in their abominations; 
  4 so I also will choose harsh treatment for them
      and will bring on them what they dread. 
  For when I called, no one answered, 
      when I spoke, no one listened.
  They did evil in my sight
      and chose what displeases me.”


But did not God tell Israelites to sacrifice animals to cleanse their sins? Wasn't the sacrifice of animals unnecessary? Why does God tell them this now?
This lamentation/complaint against Israel's wickedness and their practices of sacrifice is interesting because it is not only found in Isaiah 66 (the last chapter) but also in Isaiah 1:


  3 The ox knoweth his owner, and the ass his master's crib: but Israel
  doth not know, my people doth not consider.
  
  4 Ah sinful nation, a people laden with iniquity, a seed of evildoers,
  children that are corrupters: they have forsaken the Lord, they have
  provoked the Holy One of Israel unto anger, they are gone away
  backward.
  
  5 Why should ye be stricken any more? ye will revolt more and more:
  the whole head is sick, and the whole heart faint.
  
  6 From the sole of the foot even unto the head there is no soundness
  in it; but wounds, and bruises, and putrifying sores: they have not
  been closed, neither bound up, neither mollified with ointment.
  
  7 Your country is desolate, your cities are burned with fire: your
  land, strangers devour it in your presence, and it is desolate, as
  overthrown by strangers.
  
  8 And the daughter of Zion is left as a cottage in a vineyard, as a
  lodge in a garden of cucumbers, as a besieged city.
  
  9 Except the Lord of hosts had left unto us a very small remnant, we
  should have been as Sodom, and we should have been like unto Gomorrah.
  
  10 Hear the word of the Lord, ye rulers of Sodom; give ear unto the
  law of our God, ye people of Gomorrah.
  
  11 To what purpose is the multitude of your sacrifices unto me? saith
  the Lord: I am full of the burnt offerings of rams, and the fat of fed
  beasts; and I delight not in the blood of bullocks, or of lambs, or of
  he goats.
  
  12 When ye come to appear before me, who hath required this at your
  hand, to tread my courts?
  
  13 Bring no more vain oblations; incense is an abomination unto me;
  the new moons and sabbaths, the calling of assemblies, I cannot away
  with; it is iniquity, even the solemn meeting.
  
  14 Your new moons and your appointed feasts my soul hateth: they are a
  trouble unto me; I am weary to bear them.
  
  15 And when ye spread forth your hands, I will hide mine eyes from
  you: yea, when ye make many prayers, I will not hear: your hands are
  full of blood.
  
  16 Wash you, make you clean; put away the evil of your doings from
  before mine eyes; cease to do evil;
  
  17 Learn to do well; seek judgment, relieve the oppressed, judge the
  fatherless, plead for the widow.
  
  18 Come now, and let us reason together, saith the Lord: though your
  sins be as scarlet, they shall be as white as snow; though they be red
  like crimson, they shall be as wool.
  
  19 If ye be willing and obedient, ye shall eat the good of the land:
  
  20 But if ye refuse and rebel, ye shall be devoured with the sword:
  for the mouth of the Lord hath spoken it.


The theme is the same at the beginning of Isaiah as at the end of it: frustration.  It could be phrased in modern terms as:


  "You are missing the point," the Lord says.  "I gave you a law of
  sacrifices so that you could repent of your sins, but you aren't
  actually using it to stop sinning!  You are treating sacrifice as 'the
  cost of doing business,' as a price to pay to make sins acceptable,
  instead of as an act of contrition as part of the process of
  repentance, and I'm sick of it!  What I truly want is repentance, and
  a people who make a real effort to follow my commandments, and without
  your heart being in the right place, killing a bunch of animals really
  means nothing to me."


This is consistent with Paul's declaration in Galatians 3 that the Law of Moses was intended as a "schoolmaster to bring us unto Christ".  The sacrifices themselves had no innate cleansing power; their virtue was in the act of obedience to God's will, and in pointing their hearts towards the true sacrifice that they prefigured, the Atonement of Jesus Christ.  If Israel forgot about that, when it was the entire point of the law, no wonder the Lord was annoyed with them!