You didn't elect Bill Gates... so he shouldn't spend his money to make the world better?
Exactly.
> Are you saying people shouldn't be allowed to make the choice between those?
No, but if they have "extra money", the most moral thing to do is not choose the good cause, but contribute it to the general charity fund to be democratically disbursed.
> Are you saying the first person is doing something bad by giving their money to medical research instead of spending it?
No, I'm not arguing the relative merits of mosquito nets vs Gucchi bags. If you think this is a such a 1 dimensional optimization problem with 0 externalities or higher order effects, we're not going to have a very productive conversation.