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 Then the LORD said to him, “Not so! If anyone kills Cain, vengeance shall be taken on him sevenfold.” And the LORD put a mark on Cain, lest any who found him should attack him.