Assuming the player (and thus the character) did not intend for nor know that his counter-offensive would be fatal, or that he felt in genuine danger of his life and in need to respond with lethal force to save himself, I would not change his alignment based on this. I would, however, make sure to remind him in future situations that the last time he behaved such-and-such a way, somebody wound up dead at his hands, and watch carefully the choices he makes based on that.