I think the question of revenge, as motive for killing cannot effectively be restricted to group violence for the statement to be accurate in spirit.
After all, revenge is an individual thing, by definition no "group" can take revenge for anything, at the end of the day an individual made a choice, and that choice cannot be hidden behind the veil of "revenge for my (brethren, religion, computer club, whatever)"
There is a fundamental flaw with your original idea...
Blakenzy said:
Now I am not knocking at individual legitimate self-defense, of body and family, but at violence between social groups, nations, countries.
It seems like you are confusing violence with revenge. Violence between social groups, nations, countries, etc... Cannot be the product of revenge, retaliation maybe, but not revenge.
In the same way self-defense is in no way related at all to revenge. Self defense is about protection, revenge is about payback.
Self defense is motivated by desire to live
Revenge is motivated by hatred
BIG difference.
Again, revenge is a very hateful action, with a very selfish motivation.
Self-defense is a righteous thing, revenge is anything but.
Now, back to the original question.
I'm not sure where you got the figure that 50% of all killing is motivated by revenge, but I have a very hard time humoring that idea, it seems way too far out there.
I do agree that everyone should "turn the other cheek", that is forgive grievances, don't hold grudges, forgive and forget. I think we would all be a lot happier if everyone did that.
But then again, that is an individual decision.