Just because I think semantics are interesting, I think that he would still be an assassin had he completed his intent, simply because that act would have been an assassination. If the act is an assassination, the actor is an assassin. It doesn't mean he has to be a ninja or Jason Bourne.
RFK's assassin was a failed jockey with a head injury and an obsession with the Arab-Israeli war, and possibly drunk. Still, an assassin nonetheless.