—“Punishment for killing someone does not necessarily imply they had any actual value . At the societal level the punishment is automatic.”— Edgar Braintree 

They are not punishing you for killing the individual, but for not getting sanction from others before doing so. In other words, they are punishing you for hubris.

Getting a dozen men (jury) together to hang someone from a tree is very different from killing someone vengefully. If death is sanctioned then appeal exists, hubris, and error limited. What we seek to limit is unsanctioned deaths, without appeal, insured against hubris, and error.

If you act without sanction you are a risk to the group. If you act with sanction you reduce group risk.