What is the sin of a man who helped kill his disobedient and corrupt brother for assaulting their parents, and is there any repentance for him, knowing that he is righteous?

The rulings concerning a person who helped his brother's killers vary according to the type of assistance provided. If his assistance was direct participation in the killing, such as hitting him or throwing him from a height and so on, then retribution (Qisas) is obligatory upon all of them. Qisas is waived if the victim's guardians pardon them, and blood money (Diyah) then becomes obligatory, unless it is also waived by pardon. However, if the striking was with something that ordinarily does not kill, and they did not conspire to kill him, then Diyah is obligatory upon them, divided according to the number of strikes. If his assistance was indirect causation, such as pointing out the victim or restraining him, then Qisas is upon the direct perpetrator of the killing, not the one who merely caused it, unless the direct perpetrator is not accountable, such as an insane person, in which case Qisas is upon the one who caused it. The killers must repent and surrender themselves to the victim's guardians, bearing in mind that insulting parents is a major sin and warrants discretionary punishment (Ta'zeer) from the authorities. He may not be killed except by a Sharia ruling and from a responsible authority.