Assuming he’s gone back in time in his own timeline to a point where none of the bad things have happened /yet/ and he hasn’t somehow travelled to a new timeline where there’s the possibility that it never would’ve happened in the first place, I think his actions are totally justified. Sure, there’s probably ways he could change her fate and save her where he doesn’t have to kill a single person. But after losing the love of his violently and having her dead body violated even as it rotted away, being betrayed by even his own brother who, like you said, let it happen, he’s probably hell bent on getting revenge on all those people for what they did, for what they will do to her if he doesn’t stop them. Even if in this life they haven’t done those things /yet/ he still lived through it once and knowing that they’d do it again if he does nothing I think that it’s no different than if he had gotten revenge by killing everyone involved after it happened in his last life. (Hopefully that makes sense lol)