it's capitalism at is best you start a company in your garage take it to be worth Bilions an then give it all away, doesn't get better than this.
"Capitalism at it's best" isn't people giving away money... it's people being competitively compensated for their contributions to society-- the ability to take risks and reap the rewards.
Of course then you can argue that tax is a way to enforce a minimal amount of philanthropy onto people, forcing them to contribute to the greater good whether they want to or not... but people who seem to be very vocally in favour of pure capitalism also seem to be very vocally against high taxes.
*it's people being competitively compensated for their contributions to society*
Capitalism isn't so much the emergent market behaviour as a system that provides a means to create and develop wealth (i.e. capital). The labour market is one aspect of this, but it's not exclusively tied to it.
*Capitalism is an economic system and an ideology based on private ownership of the means of production and their operation for profit.*
The guy is a ruthless, greedy arsehole.
But let's suppose for a moment that he was one, and he hasn't changed, and he still is a terrible person underneath. Does that make the people who would have got malaria but didn't because of the Gates Foundation's efforts any sicker?
Or, contrariwise, consider some area where it turns out that the work of the Gates Foundation has done harm as well as good. Education seems like it might be an example. Let's suppose that in fact Gates is a total saint, motivated by nothing but pure benevolence. Does that make someone whose education was worsened by the foundation's activities any better educated?
I find I don't really care very much whether Gates is a good person, for any particular definition of "good". I care about the effects of what he does. Maybe he's giving his money away to make himself look good, or for a bet, or because he hopes it will placate an angry Microsoft-hating god, or something. Who cares? What matters is what actually happens as a result of his giving that money away.
On the whole, that's looking pretty good.