It is at its core, the issue of the perfect being the enemy of the good. Bill might be an imperfect angel, but he is an angel nonetheless in spending his fortune to help millions/billions out of abject poverty and suffering. That dwarfs almost any effort by other people.

You could psychoanalyze this as acts of redemption, guilty conscience, seeking further world domination or whatnot, However it's undeniable he's helped many, many people and far, far,far more than his keyboard detractors ever would or could. And that's a very good thing IMHO.