The problem I have with Bill Gates is essentially the broken window fallacy. Microsoft had a stranglehold on the computer industry. They held back the computer industry by years. A decade even! In the web development industry alone, we're still shaped by the impact Internet Explorer 6 left - which was released in 2001, twelve years ago. What economic value would Be Inc. have brought? What charitable donations would have resulted from that company? Or Netscape? Or any of the tens of thousands of companies Microsoft had a deleterious impact upon?
He has since spent some of the money he earned holding back some of the most important industries in the world trying to help people. That's good. He didn't have to do that. But people judge that work as if it stands alone. It doesn't. The work he does with the money he has comes at the expense of the tens of thousands of companies that were held back or destroyed by the illegal and monopolistic actions he took.
I think it's unlikely that, economically speaking, the actions he took were a net win for society. Yes, once he had the money, he did good things with it. But the cost to society for him to obtain that money is far too high in my eyes. Higher than the value he brought.
Essentially, he took it upon himself to be Robin Hood. He stole from the rich - Western society - to give to the poor. However I don't see any reason to believe that the theft he perpetrated - the value he stole from society - was less than the value he brought to society. And I don't think it was his right to commit those crimes to do those things.
By all means, judge him for what he has brought to society. But you should not do that without judging him for all that he has took from society as well.