And in order to stay rich, you have to play your role and participate in a society that oppresses the poor which in turn maintains your wealth. Are you really still capable of being a good person?
It's a complex topic, but this is the crux of the matter I think. If we're talking about today's world, then I don't think there is a billionaire who is not complicit with participating in a system that is rigged (for lack of a better term) in their favor, and profits in an unfair scale from the work of others.
On the other hand, I also don't think you need to disown your material wealth and start living paycheck to paycheck to be able to qualify as a good person. So... in my head, there's definitely millionaires who are good people, who earned their riches with authentic hard work and some genius ideas/inventions/services, and pay the people they employ well, or keep good relations with the people they work with.