But if you're going for some sort of obscure criminal achievement by only breaking one law at a time, then just make your lethal device something that does not legally qualify as a firearm. A black-powder muzzle-loader or a Bulgarian ricin-umbrella would do the trick, depending on whether you care about noise or not.
Or, like the parent mentions, make a grenade, and deliver it via remote-piloted aircraft.
Realistically, though, very few people on this planet would bother with any of that when they can often just kick down the door of your home, kill you with a tool purchased from a hardware or sporting goods store, and just leave before police can respond. The clearance rate for violent crimes in the US is already low enough, even for people who give no thought whatsoever to covering their tracks.
I'll continue to assume that unmanned aircraft are carrying cameras, and assume that if someone wants to murder me, they need a compelling reason to have a more complicated plan than to just bash my skull in with a big rock when I'm not looking.