The World's Finest Assassin Gets Reincarnated in Another World as an Aristocrat, Vol. 7 (light novel‪)‬

To Kill A Comrade!

Naoise has turned his back on humanity and become a servant to Mina, the snake demon. Lugh thought he could find a way to stop his way ward friend without killing him, but things come to a head when Naoise turns his sword against his own people. The world’s finest assassin has slainwicked aristocrats and monstrous creatures alike, but this is an opponent unlike any other—one he cares about.