God created the Universe.
God created people, to live in the Universe.
God created Satan.
God gave Satan power in this world.
Twice Jesus, Himself, called Satan the ruler (or Prince) of THIS World.
When Jesus was asked if He was the King, He said: "Not of THIS World."
So, Jesus is part of the OTHER World, the one where Heaven is.
People have a choice:
1) Try to focus on God, and follow HIS ways; or
2) Follow the "warped" ideas that Satan places in our minds.
The question: Why did God allow the Holocaust?
Why did God create EVERY person so that they would die some day?
Because, that's what God did.
Each person who is born, will die (and hopefully move on to the Afterlife).
This Holocaust question is really Bogus.
The same person can ask: Why did God allow the Civil War to happen?
Or, Why does God allow children to get Cancer and die?
God gives people the choice to live whatever Life they will Live.
If a particular person happens to be named Adolf Hitler, then he can live his Life the way he chooses to (just like the rest of us).
If this Hitler-guy gets given control over a country (like Germany), then he gets to make policy decisions about how his citizens live (or die).
In the case of the Holocaust, Hitler decided that he would like to have LESS Jews living in his Country.
He, as his country's leader, decided how to enforce his whim: "Hey, why don't I just kill-off all of the Jews?"
Because other people in his government obey their "king" ... they followed his orders.
A few years later, 6 Million Jews were dead.
It's about as simple as that.
God does NOT prevent all evil.
I am sure that God would prefer that people do NOT commit evil acts.
But, God has given us Satan, to confuse us, and cause problems (like the Holocaust).
Each person has the Option to be nice. This is what's important: What I do with my Life.
Other people have their own Life, and I hope that they will act nicely too.
But, the reality is: Many people do awful things.
My job, as a Christian, is to accept the things that have happened, and try to make a better World (in spite of it).