Last hit could have gone down badly

First he kills the guard by putting tape on the glass then shooting through it. But, for the last guy he goes down into the basement. Then takes off the drain pipe. Then gets his attention with his laser. Then he just happens to line himself up perfectly for the head shot when he leans over the sink. What if he didn't decide to look down the drain? What if he looked down the drain but then pulled away before he could get a perfect shot? Now he knows some assassin is in his basement. Now he can barricade the door and grab a machine gun. It seems like such a one in a hundred shot. So many ways it could go wrong. It's hardly the failure proof plan that a cunning and witty samurai would come up with. I think it was done just to make him seem like a killer who would think outside the box. Or just to seem like some kind of unorthodox sneaky assassin. I think that could have been rewritten to something different with a fail safe. Something that didn't require random chance to work.