not really what's happening here. This was a demon that he shattered the core of when he was teaming up with this girl (the little sister of the other swordsman in the hero party who, along with the saint, was questioning the hero's accusations about the mc and then died protecting the saint when it turned out that the demon king's head they took as a trophy was still alive). He didn't leave this demon alive out of mercy, he broke it's core into pieces and one of those shards was enough for it to possess her.