I refuse to believe that the loss of life is perferable than the preservation of life. And quite frankly, arguments from classical economics seem rather cold given the subject matter.
The heart of your argument suggests that the political issues in these countries are the root of the problem. This is correct. However, history shows us that the amount of money spent on politically bankrupt states has little or no impact on making them politically sound. Gates money is useless on the third world political playing field; instead, Gates is choosing to spend it in an area which he can actually have an impact.
As far as domestic politics go, the positions you suggest that Gates take on "farm subsidies, fossil fuel subsidies, and the recreational diamond market" are all politically untenable at the moment. It's not clear Gates could have an impact there either.