Is the execution of a rebel mandatory if they kill someone from the peaceful populace outside of battle?
General Chapter
Al-Mughni
Book of Fighting Rebellious Groups
Primary text
There are two juridical opinions on this. The correct position is that execution is not mandatory. This is based on the statement of Ali (may God be pleased with him) concerning the matter: If you wish, you may forgive, and if you wish, you may seek retribution (qisas). For the Kharijites specifically, their killing is permissible (ibaha), and thus there is no retribution against the killer of any of them, nor liability for compensation from their wealth. No captives are taken from them, their wealth is not seized as spoils, and their descendants are not enslaved.
Supporting text
One view holds that execution is mandatory because the act constitutes killing through brandishing weapons and spreading corruption in the land, thus mandating the death penalty, similar to a highway robber.