What is the ruling on someone who kills a person who tortured them for years?

If the slain person’s blood was lawful to shed, such as a combatant from a belligerent nation or an apostate whose apostasy has been confirmed and whose blood has been declared forfeit by the ruler, then there is nothing upon the killer. However, if the case is otherwise, then the killer has committed a grave sin and a major crime, and he must repent from it. He must also surrender himself to the victim’s guardians so they may choose between retribution (qisas) or blood money (diyah), this being if his killing was intentional and aggressive.