Why this criticism? Is HIV research not a good enough cause for you? Much better than a 10 billion dollar airport or 15 billion dollar Olympics in my opinion.
He may go down the evangelical Christian path and fund a war of sex, abortions, gays, trans and people’s reproductive rights. That’s not even a hypothetical outcome, there’s plenty of millionaires and billionaires (like David Green) already funding that war.
But also, there's no guarantee that _anyone_ including the government will use money for "good" causes. Bridges to nowhere, military adventurism, wars on drugs, personal enrichment of politicians and government employees, vanity projects of various sorts, the list of things governments mis-spend on is vast.
And of course we don't all agree on the relative merits of different causes.