You can make an IED with easily-purchased materials, if you know how to do it. There are guides circulating on the internet.

It isn't that hard to learn to shoot well enough to hit a target at 200 feet. It takes 1-2 weeks for a person of average ability. Allegedly, Antifa is offering sniper training classes. Most celebrities and politicians don't have security teams capable of sweeping all the rooftops in the area. Only the President and Secret Service are that thorough. A couple of billionaires can afford security that good. Bill Gates and Elon Musk probably have security competent enough to stop a sniper.