An assassin should be killing for pay. I would suggest that the th e significant part of RP needs to be betwen the victim and the assassin's hirer. As folks have mentioned, the RP before death is very important. The victim should be aware that someone is out for their blood; should have a significant level of fear. The Assassin and Hirer should also be RPing in such a way that there is a chance that they can be found out. IE, all negociations done IG rather than in PMs on the forum. I'd probably go as far as saying all communication should be IG. This can be done anonimously with go-betweens, written notes IG and stuff. PM's prevents the sheriffs or the victime any chance of busting the strike before it happens.
The actual strike, I think, could go ahead with very little RP. After all, an assassin worth his salt should strike first from the shadows with a very significant strike, and then flee. An assassin that gloats over the corpse will die, and quickly. If the RP has happened between Hirer and victim over a week or two, I think folks will understand and appreciate the whole game.
Having said that, assassinations are always going to be a challenge to do well.
Sounds right to me.