Self preservation is at the root of it; "If he's dead then he can't hurt me."
Also, revenge is a basic human instinct.
Just because we have police and courts these days does not mean we've lost our animal behaviors from the 250,000 years before the modern age.
The modern court system isn't to protect us, the public, it's to protect the accused until he/she is or is not convicted - and then the correctional system is to protect the public.
We are all animals at our basest.

Kerbal Space Program:
Failure is not an option. It's a requirement!

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