You don’t have to register it with the FAA; you’re already going to murder someone, who cares if you break a few more laws?
The last point assumes someone’s recording the wireless activity of the drone in the moments before the explosion, which I think is pretty unlikely. And the internals of the drone should be destroyed by the explosion, rendering it practically impossible to extract any identifying information other than the general drone model.
And even if all wireless traffic is being recorded and triangulated, pick a busy place and you’re just one guy on your phone in a crowd of thousands. You can also order the drone anonymously months ahead of time and pick it up somewhere with poor security camera coverage to all but ensure there’s no record linking it to you.
True, though you’ll probably be recorded by security cameras if you buy it in a physical store.