A betrayal and it's aftermath. What exactly happened in the dark?

Ever wonder how much difference one person could make? A person who has seen one way things would go, but thought how would things go if he could try and change things. Yeah, this is a Self-Insert, but please read through it, it has a S-I with the Gamer power, along with a little something from Boku no Hero Academia. Rated T for stuff that happens later on in the story.