I doubt the first (civilian) drone assasination will be that sophisticated. Face recognition from a moving object at an unfortunate angle is not easy and it's even harder if you want to do it from a fixed-wing drone that can't hover like a quadcopter. I'd rather expect an assassination by a fixed-wing remote-controlled UAV targeted at someone speaking at a public event, some place where traditional methods (knives, rifles, car bombs) fail because line-of-sight access is heavily restricted.