If I may, I would like to give you a condensed, nutshell summary of what the Bible says about such awful things happening, not just the Holocaust, but all other bad, horrifying things. Please bear with me, it's not a short story.
The reason is that when Adam and Eve were first in existence, all was well in their Garden of Eden. But humans and angels both were given free will as to whether they continue to comply with God's requirements and standards or not. God had the right to set those standards, being the Creator.
One angel got some ideas about having a 'better way' and used the serpent to talk to Eve and get her to thinking like he was, and we know that she did eat off that tree, thereby disobeying God, and Adam ate from it later too so therefore, humans were officially messed up and imperfect. God withdrew his help to them and they began to break down mentally and physically and it would get worse thru the ages.
God did not destroy them or the bad angel right then because the angel had raised some questions. If you look at Genesis 3, the first few verses, he questions Eve about what God said and then just outright lied, 'you positively will NOT die'.......The first lie in the whole universe.
And everything went downhill from there. But this lie called into question whether God told the truth about anything, and it also questioned his way of doing things. Big issues, right?
So the wise thing to do was to let time go by, let that angel try to prove if his way really was better. He promised Eve she would become like God, knowing good and bad.
In the book of Job, there is more information about this very issue, it's so interesting. Chapters 1 and 2 tell of Satan (the bad angel) appearing before God and challenging him.
Anyway, this account shows that Satan now included all of mankind, not just Job (ch. 2 verses 2 thru 8). We are all involved in this situation now. So God allows more time to pass, enough time to prove whose way is truly best.
We have been living Satan's way for thousands of years, what do you think? Is it working? His way is the way of war, violence, terror, cruelty, and on and on. That's his way of thinking, and that is why horrible things have happened for so long. However, God has not been idle; he has a purpose for the earth and he has had to take a detour in that, but he will still achieve the end result of a beautiful earth with good people on it living in abundant peace and real love. According to Bible prophecy, that time of change is very near.
I'll share a few Bible verses that describe Satan and then some that describe God and you decide for yourself who has the better way.
Satan
Hebrews 2:14, 15 1 Peter 5:8 Revelation 12:7-9 Job chapters 1 and 2 Matthew 4:1-11
Jehovah God
Exodus 34: 6, 7 Deuteronomy 32:4, 5 Job 34:10 James 1:13-15 Romans 8:31, 32
If you would like more information please go to jw.org.