First of all, Billionaires don't have their wealth in a pool of gold like some people believe, it's invested somewhere, generating jobs, giving value to people. So if you tax them a lot, you're affecting all the people working for them, etc.
This goes along the idea of the "Cake". The money is a fixed cake, a sum 0 game, and if they have more, I have less. That is also wrong.
If someone discovers a cure for cancer, and becomes a billionaire from it, I wouldn't mind. After all he's benefiting everyone.