you get a weapon for killing nobody that shows up at the very end of his chapter. like, after you would have a chance to even use it.
not sure where you're at in his chapter, but if you're not very far in, I recommend just playing normally unless you really want him to be a pacifist for roleplaying reasons, which, fair enough. probably don't do it for the weapon. killing every enemy or killing no enemies are both tedious puzzles that's meant for repeat playthroughs. weapon will get outclassed pretty quickly by the next time you have access to oboro.