That's an excellent question and I've been wondering about this a lot. I've known a few billionaires, exactly one was an amazing person, the rest were absolutely terrible people in every sense of the word. The whole word is full of NPCs to them. But your question goes to the heart of it: were they terrible people because that's a requirement for becoming a billionaire or did becoming a billionaire somewhere along the line turn them into terrible people?

I haven't found the answer. But I do know one thing from my (smallish) sample: the nice guy never let the money get to his head. He drove his own car (a VW Rabbit, ok, a nice one) and lived in the same house that he lived in when he started his company. And until the day he died he just kept doing what he loved without ever feeling the need to lord it over others. More like a Wozniak (who is the other 'nice billionaire' that I'm aware of but I've never met him).

Edit: I wrote about him:

https://jacquesmattheij.com/in-memoriam-rene-sommer/