Stupid idiot
How utterly stupid. In all seriousness, I hope the attendees sue him. He voluntarily and willingly inflicted fear and emotional distress to all of the attendees, allowing them to believe that their safety (and, in fact, their lives) were in danger. That is completely unacceptable behavior, even for one of the richest men in the world. If he had said, as he released the mosquitoes, that they did not carry malaria, that would be one thing, but he did not. There is also no guarantee that the mosquitoes weren't carrying any other disease which the attendees could not be infected with (much like deer ticks can carry Lyme disease). To say nothing of the possibility of one of the mosquitoes sucking the blood of an HIV-infected attendee, then introduces that blood into another attendee. Sure, in all likelihood, there will be no negative affects other than some temporary itching, but the truth is, we (and he) just don't know.
And, since this is the US and the previous administration liked to call everyone a terrorist, consider this -- Bill Gates really is a terrorist. By definition, a terrorist is someone who uses terror/fear to coerce someone into doing something. And that's exactly what Bill Gates was attempting to do. So when will he be arrested and tried as a terrorist?
There are plenty of (correct) ways to get your message across, and to ask for aid. That was not the way to do it. And while a lot of people will defend the guy, if he's that gung-ho about it, let him contribute a lot more of his personal fortune. He can afford it a lot more than those attendees.