This is why there’s that trope where the bad guy gets killed in the process of, or just after, getting redeemed. So the story can have its cake and not have to deal with any of the icky justice afterwards. How jarring would it be to have the bad guy turn around, save the day, and then the heroes still kill them or drag them off to a trial for their crimes? So justice has to be meted out by fate rather than having to complicate our heroes.