Okay, that wasn't clear why Alex wished to go back until the reveal, because it seemed like he wished to return for the sake of his wife... not "revenge" for his wife murder he pushed onto her.
Recap time then.
Rose, Alex with their children are living with Alex bedridden father who's illness drained the little income Alex made.
Unable to choose, Alex pushed the hard decision on Rose to kill his father, she chosen a way to save the four of them but took her father-in-law life to look like he passed away "naturally"...
Concluding. Yes, Rose commited a murder but she was under the wall, either let them all starve and die, abandon father-in-law in the woods where he would die of starvation or kill him in mercifully way, but as Alex didn't have guts to pull it end his life quickly (arguably, that's very hard thing to do to own family members, especially parents), had to unfortunately kill father-in-law more painful death than quick but obvious to Alex.
Rose soul shouldn't technically be tainted IMHO... think Raul fucked up the judgment here, if he knew the reason why she killed Alex father.
Thanks for the scanlation!