The issue is that any philantropy a billionaire does comes from money "earned" through exploitation and is never enough to un-make them a billionaire. Even if they did, it's still a single person taking the resources of millions of people and controlling it themselves to put into their pet projects, in a completely undemocratic manner - so Gates gets to benefit from the looting of Africa and then turn arounf and tell Africans how he will be allocating that stolen loot. Oh, and that man controlling so much policy in various African nations thinks Africa is overpopulated, an extremely racist eugenicist myth.

The good and bad are not separste things you can judge in isolation, any "good" a billionaire does is only possible by causing disproportionate harm. It is not as though these billionaires are personally doing much of anything, they are simply seizing resources from the public to inefficiently address problems that the public could have managed themselves if they were permitted to control their own lives, if they aren't just doing what Gates does of using donations as money laundering.