Actually, he's funding the work, not curing Malaria himself. And he's doing that in order to redeem his legacy, something that wouldn't be necessary (redeeming his legacy, not curing Malaria) had he not made Microsoft in the first place.

And, BTW, he didn't invent this idea of amassing vast resources with shady deals and then becoming a philanthropist and redeem himself with this. Andrew Carnegie did it first.