You know, Hitler wasn't a bad person at all at first. He did a lot of good things for Germany, he created national policies, a heathcare system among others, that are still used today.
What led him to snap was injustice in fact, where he developed a disliking for Jews, and his position of power ultimately gave him the ability to let himself be turned into a maniac.
The scary part is that such could happen to any one of us. All you need is a person or a group of people pushing your buttons repeatedly and continuously, putting you in a vulnerable spot, and then having them be mean and unfair to you by attacking your psyche and your heart repeatedly and continuously, until you finally snap. Every human being has a breaking point, they say.
And every person has a monster in them. It's just a matter of pushing the right buttons to get it out.