This is probably a slightly misguided idea to go after them as bad people because as soon as they do do something "good" you leave the door open for people to think that perhaps on balance they're not so bad after all.

The problem of billionaires being billionaires is itself the chief complaint people should have. It doesn't matter if they're Mr Rogers and Santa Claus combined, because they can choose to be so entirely at will and can be selfish assholes too entirely at will. They can also be other things entirely, given they are actually human beings after all they can try to act on best intentions, but like all humans, with great ignorance or with flawed thinking. When you or I do that the consequences can be terrible, but mostly, we'd be unable to come close to the scale of impact these demi gods can leave in their wake, not to mention the "original sins" that allowed them to become billionaires in the first place leaving a legacy of nasty indirect consequences for society at large.

There's actually a lot of examples of billionaires philanthropy and as you likely expected to point out when people mentioned that, some of those acts hide less pure intention, but undoubtedly they probably really did do some good and that itself is enough to completely undermine your whole point that they never do anything good. The issue is that, with the sheer vast quantity of concentrated wealth and power they can wield, the society that supports them is bereft of a real voice in how it's resources are used. So much of the fruits of our labour end up closed off in private coffers and it undermines public institutions like democratic governments because while we may theoretically have a say in what they do, we legally have no say at all in how a billionaire spends his bucks (and I say his intentionally). They might say we oughtn't since it's their money and no one typically has a say in what the rest of us do with our money but as with most things, there's a point of extreme where this logic becomes perverse.