Ratskinner
Adventurer
A D&D assassin should be refluffed into people trained in various methods of killing others and the surrounding abilities needed to get to the kill; information gathering, stealth, bypassing traps and wards, opening locked doors, indentification of people and place, tracking, and disguise.
That way players can create hunters of a faith's enemy, a king's counterassassin, a VIP's secret security, crime fighters who cannot risk being known publicly, a protector that deals with threats that cannot be dealt with fairly, and of course hired killers.
Sounds like a 3e rogue with some feats, to me. I mean, call yourself an assassin if you want, but....
It's not like an assassin class will keep me from buying the game, though.