Theodicy is the attempt to justify how an omniscient, all-powerful, benevolent god could allow such apparent evils like the Holocaust. Many have tried to explain theodicy but it is a bit like getting the square peg in the circular hole.
Theodicy can be explained by saying the world is as good as it can be. This seems to get god off the hook but only if you are inclined to do so anyway. This kind of explanation might have worked when humans needed gods to explain the existence of the sun, floods, earthquakes, and so on. But we have natural explanations of those and yet we still have to explain what appears to be an excessive level of evil.
For me, it isn’t just that horrible events occur which you would think would not occur if there was a god. Worse, is that many of these events occur in god’s name. Hitler thought he was doing god’s work. The perpetrators of the terrorist attack on 9/11 did likewise. Sorry if this seems too political but how can we explain all the very religious people who made Donald Trump president? If god exists, he seems to be otherwise occupied.