The problem with billionaires is that they are able to hoard so much money by exploiting others. We would be much better off if billionaires weren't given so much advantage by Capitalism as those resources would be much more useful if distributed.

The biggest problem we currently have with billionaires is that they are now so rich that the world becomes like a game to them and some of them are deliberately pushing us to a dystopia where non-billionaires become functional slaves (c.f. Amazon workers).