I believe the point is that it takes (1) person to end an abuse cycle. He's a shit person since he made the choice to abuse other people. It was never an excuse, which is the point. There is NO excuse for the shit. It just explains why.So the diary at the end appears to be that scumbags. I don't feel any pity though. Not that he deserved the mother he had, but it's not even close to an excuse for his character.