The previous offenders were all his family and friends, who went about it the wrong way but ultimately had good intentions. Would you have cut all connections with your family and friends, unable to move on all alone and miserable, if you were put in his shoes? Family is complicated, every family has a different dynamic, but in the end, they're all you have. Luckily, MC's family members are good at heart, and that's what matters; not everyone is blessed with a good family (as we can see in this update, with Amanda as well).
His family may have wronged him, but MC isn't a saint either. He would have been killed by the powers that be if he continued down his twisted path of justice. Even if they did something terrible, his family managed to save his life in the end. If you can't forgive your family for something like that, then maybe you won't be able to forgive anyone at all, then the problem truly lies within you.
I feel like he should have understood his family's position more and been more empathetic towards those around him, after realising his own mistakes. Imo, it shouldn't have taken him so long to forgive his family in the first place, but he's also a flawed character, as messed up as he is, he took his own time to process and ultimately forgive his family. Then again, if it were me I would have never got to the position he's in, I'd have done things differently. So I can understand MC's character as well, in this whole narrative.
This isn't a revenge story, it's about the MC's redemption and reconciliation with his family; because in the end, he also cares about his family, and wants to protect them. Finding and dealing with Stanford Nelson is the last thing that MC needs to keep his family safe, so no one comes after them anymore, and they can live in peace.
While I disagree with your opinion, you're allowed to have a differing one, be it right or wrong.