So if part of a group commits a violent crime, the whole group should suffer violent punishment, up to and including death.

If one man is a serial killer, his whole family should be killed.

If a a school gang kills somebody, the whole school should suffer violent reprisal.

If a gang from a neighborhood kills somebody outside that neighborhood, others from within that gang's neighborhood should be killed.

If a policeman murders a person while on duty, it's right and proper for people to kill his fellow policemen in retaliation. Gotta make violence untenable to aggressors, y'know.

See how that works? Cruelty begets cruelty. The desire for vengeance does not magically go away, not until one side reaches out to show unasked-for mercy and compassion. We did that with the Marshall Plan.