Earning a tremendous amount of money and then amusing yourself by spending it on "charity" for the rest of your life doesn't make you a good person.
It's just one more method of buying good feelings and trying to buy good will while being in control of large numbers of people.
He wants to feel like he's doing good and using money to give him that feeling.
It's like you're allergic to subtlety. Yes, saving untold numbers of children from malaria is a good thing. You can do bad things and good things and while everyone else is arguing about morality, the thing that matters is the end effect. Did Bill Gate's time on earth result in a better world when he's gone or a worse one? I won't pretend to know enough about his life to answer that, but he has prevented a lot of really, really brutal suffering.