I think it's not that. In the last few panels, MC decides that he shouldn't kill players because that's Souta's body and not his own. He says that if he kills another person using Souta's body, Souta will be the one suffering trauma once MC finds a way to get back to his own body in the future. That's why he shouldn't kill players. In that last scene, we see what looks like MC, but you can see that his eyes are not human and he has horns on his head, which implies that's a demon. It's possible that this is actually MC's real body, which may have been transported also to this world (perhaps as the Maou) as another demon character like that other player who became a demon. The question would be who's in MC's body as his soul is in Souta's body.

I assumed it was the MC because it's the same face as when he's in his thin form. When he killed the first boss monster that almost killed his sister, he mentioned that it was a player just like him acting like a mob, and that the curse they have in that world intensifies when they follow the script forced upon them, so it could be a mutation caused by the curse. If it really is the MC's original body, then it would be more logical to be Butao's complete consciousness in his body, which would make sense of the unfinished sentence to the girl who killed him.