hmm ....
Orochimaru is too hard to motivate -- he's just there to have fun.
Gaara is too hard to control -- he loses himself to whatever lives inside of him, making him unreliable.
Kenshin "went soft" so he'd probably be bad.
Killua was born an assassin but I don't think would kill without a good reason anymore.
Jiraya would rather see more ero than waste his time killing some random stranger.
gattsu is too busy with his own vendettas and his quest to find a way to live, so I doubt he'd take the job.
Jubei does specialize in mass slaughter, but he's got a noble/honorable aspect to him that makes him not a coldblooded killer -- your target would have to attack him or someone he cares about enough to save.
And then there's the dbz crew ... they're not exactly stealthy. "kill this person" doesn't require "blow up the world" -- on top of that, the bad guys are mostly all dead, and the good guys are mostly not coldblooded killers anymore.

I think I'd have to go with either Alucard from Hellsing (combination of insanity, lack of regard for most humans, stealthy sink-into-the-floor powers, and the whole immortal vampire lord thing), or maybe Kyo from sdk (also crazy, also uber-powerful, not immortal but might as well be). Kyo is a guy who has no regard for life, and is ultra-skilled and ultra-powerful. I think he'd work just fine.