I don't think that the cheating of the mother actually "transformed" the MC. He is a drugdealer and not the nice guy from next door. I think he always had this unhealthy desire for his stepmom/mom whatever and he uses her cheating as an excuse for himself to defend his actions. It is pretty clear that he is a bit crazy and if you read carefully you will see that often reflected in his inner monologue. There's one thing the game does really well: showing that what the MC thinks, the mother/stepmother actually is thinking/feeling and how they act is not always the same.