From the first chapter, there are 2 ideas that contradict themselves: 1. this is war; 2. not killing the enemies even though it is killing your allies.
Probably the former hero still confused and still cant kill his former ally and maybe in the next chapter, it will be different.
But then in the second chapter, he still dont want to kill the human that you cant reason it, like warmongering people (if he spare the heroes that still have decency then I can still understand him).
I hope the next chapter, he can become more of a leader for the demon clan. As a leader, you need to take a harsh decision, like killing for the sake of saving a life. I mean you cant defend the demon clan while the human (and god?) want to kill you, but you choose to spare them, let them recover, and let them try to kill you next time. Maybe there is diplomacy later? I hope so, rather than he overpowered all of the heroes by himself and this manga become your trashy OP protagonist manga.