@Christopher
I always just put down his change in personality to the fact that he did his best to help people and in the end it seems to have literally gotten him killed and then revived as something not human (complete betrayal by comrades and becoming inhuman = change in priorities, it seems reasonable to me)
regarding the inhuman part, the chapters are gone (for now) but, in the icelands red gate event the monster boss said there's something like a voice in their heads telling them to kill humans, but that it didn't happen when he saw MC, just the other humans in the party