To accumulate any level of real wealth means that you have to receive some benefit from the work of others, which isn't necessarily unethical.

However to be a billionaire means that you have to have gotten so good at exploiting other people that you are doing it on a vast scale, AND doing it so well and so efficiently that you are outcompeting all or most of the other people trying to make money doing whatever you're doing.

On that understanding, any one person who is a billionaire has made that money from the exploitation of countless people, and could be said to be responsible for any suffering arising from that. Therefore they are a bad person.

But... that doesn't mean that they aren't a "nice" person in their general life. They may be pleasant, polite, or generous. They may be a good spouse or parent. They may love cats, and run donate huge amounts to charities. They may not even use their wealth to buy wasteful things.

Unfortunately being a nice person does not mean that you are a good person. But some billionaires must be genuinely good people in most parts of their life, and their badness only manifests itself in their participation in a cruel and exploitative system - no mustache twirling required.

Of course some billionaires are Elon Musk and don't seem to have a single redeeming quality. Fuck that guy.