I remember this isekai manga that had a short side story where the king just went out and killed the demon king but kept his head to keep as a friend. If possible what is it?
This is Benriya Saitou-san, Isekai ni Iku, aka "Handyman Saitou In Another World". The main storyline concerns the titular Saitou, who entered a fantasy world from ours. Completely unrelated to this, in Chapter 11.5, a king posts an ad for a hero to slay the demon lord. No one shows up, so he just does it himself and brings back the demon lord's severed head. The head is still alive, and the king plays chess with his new "friend".