Whoever becomes a murderer does not go unpunished. The Old Testament teaches the death penalty for the murderer (Genesis 9:5). However, no one is allowed to simply use that punishment. God himself remains the judge (and executes justice through persons whom he designates, Romans 13:4). Thus, God puts a limit to arbitrariness and revenge. The endless stream of blood, which flows from this first murder over the world in all kinds of forms, God wants to channel through justice and righteousness, without downplaying the evil of the shedding of (human) blood. Justice can never be simply applied. It is preceded by a (judicial) judgment, which is here pronounced by the highest Judge himself. The death penalty is not (yet) an issue for Cain.
15 L'Eternel lui dit: Si quelqu'un tuait Caïn, Caïn serait vengé sept fois. Et l'Eternel mit un signe sur Caïn pour que quiconque le trouverait ne le tue point.