Bill Gates didn't become rich because Microsoft managed to work itself into a monopoly position with dirty tactics. The whole problem people have with Microsoft is that they abused their monopoly in one area to gain a foothold in another! They already had a virtual monopoly on desktop computers. How did they get there?
By having smart people and making really good software that more or less worked everywhere.
When Microsoft started becoming evil they were already fully entrenched in the market.
How many millions of US$ would those people have donated to charity if their budgets hadn't been constrained by purchasing Microsoft software? Some of them were probably African countries that could have used that money to buy vaccines 10 years ago.
Yes, I'm sure that any money left over from a large corporation's IT budget would have gone straight into the donation budget.
Microsoft got into its initial monopoly position because of an "interesting" deal with IBM. Specifically Microsoft got a contract with IBM to deliver a product, turned around and bought the product from another company that had already developed it, and then delivered that to IBM. Microsoft then hired the person who developed said product because they didn't actually have the expertise to support it themselves. See http://inventors.about.com/library/weekly/aa033099.htm for confirmation.
About charity, Microsoft may have improved, but a few years ago standard operating procedure was for the Bill and Melinda Gates foundation to tie charitable work to large Microsoft purchases. If a government bought from Microsoft, the foundation would come in and start providing assistance. If the government went to Linux, the foundation would pick up and leave.
The use of this tactic has left me with a bad impression of said foundation, despite all of the unquestioningly good work they do. (They may have gut this out more recently. I no longer pay attention to Microsoft now that it is clear that they aren't going to be able to destroy the parts of the IT world that I care about.)
Its pretty hard to say that the only way MS leveraged its monopoly was to enter other markets. That may have been the biggest problem, but I doubt it is the 'whole' problem. You really believe that MS didn't gouge consumers on the price of their OS? Or their Office suite?
Regardless of your opinions on their specific business practices, I think the OP's point still stands: its difficult at best to laude someone for giving away money that to some extent was derived illegally.