I'll continue presenting the Religious Nut point of view (note that I am not a Christian/Muslim/Jew and don't believe in any of this):
You ask why it would be part of the plan of a good God, when you should be saying, 'If the Holocaust is part of Gods plan he is evil'
- But God is perfectly Good.
- How do we know he is good?
- We can see it in the beauty of His Plan.
- But wouldn't God's plan include the Holocaust?
- Because when Adam sinned, etc.
God's already deprived someone of Free Will to show off his abilities, see the Pharaoh.
- That was a one time incident and an exception was made for whatever reason.
So why did God create Satan?
- God created Satan as part of His Plan.
- So God had a plan that he knew would result in suffering.
- But God's Plan could only exclude suffering if he eliminated free will.
- But it wasn't the will of the Jews to get mass murdered in concentration camps, etc.
How did they turn away?
- They turned away when they chose to commit sin.
- But why did all this bad stuff happen because of "sin"? How does that follow?
- God's perfect Justice requires that the wages of sin is death, etc.
How is everyone a sinner, how has everyone sinned, it's statistically impossible for everyone to have sinned.
- God's definition of "sin", in addition to all the regular "bad" things, also includes bad intent, so if you desire to do something bad, you've already committed the crime according to God's "perfect justice".
How is it 'Justice' if you're punished for victimless crimes for all eternity?
- Because that's the way God said it was and God is perfect, so we can't question it.
If God's already planned everything out then he's already denied people free will and is forcing them to commit evil and punishing them for something HE made them do.
- But, you see, when God created us, he gave us the magical gift of Free Will (tm), so that even though we have free will, God still knows what we are going to choose and so we are still following God's Plan (tm).