The internet love to rib on billionaires, but there are some who are genuine. Like Charles Feeney, who made billions from running Duty Free during the golden age of air travel. He secretly donated much of his wealth and his worth was only revealed during a legal battle when had to disclose his finances. Right now, Charles' net worth is said to be now in millions since he donated much of his wealth. If I remember correctly, I think he always lived in a rented apartment too. But I think the type of billionaire like Charles Feeney qis far and wide among the group.
To be honest, I think saying that being a billionaire is unethical is farfetched. If someone made that much money running a business ethically (assuming that's what Feeney did), then I don't see why anyone should feel guilty for becoming wealthy. And the guy is old, in his 80s now I think, so he knows that there is no point hoarding all the wealth he accumulated if he knows that his time will come soon.