That is exactly what Darwin disproved. The survival of the "fittest", not the strongest.
Humans are not strongly, but can adapt quickly.
If you are attacked in PvP and want revenge, well, technically spoken you can't keep a track of "has attacked and not yet sought revenge" for it. it would be loooong.
Also, it would most likely kill PvP.
Currently you can haunt the guy you want revenge on, you can put a bounty on him, and (that is the main purpose of clans) send someone from your clan after him.
What else is necessary? A "kill him by poisoning his food"?
It would lead only to a general "all slay all"....
or many pick on a few.