Since it ends like that, I guess it's acceptable that this is a short story.
I don't think the story can stand alone. Because people wouldn't know how he, the loyal assassin could suddenly switch loyalty and how the torture actually brainwashed him.
This is a bit average don't you think? Of course, you still have the imagery and all that in line. And it is still good since if you were to write a long winding tale of the assination, that would probably be a bit of a problem. So anyways, nice one here.