Without a legal system, what other form of recourse do you have?
Obviously, murder is a very serious form of retaliation, but with no path for de-escalation (a trial, a judgement by a religious, or community elder - or a feudal warlord), eye-for-an-eye will eventually reach that point. Let's not forget that most cultures, including ours, prefer, or at least tolerate revenge over turn-the-other-cheek. It's why it's so easy to treat criminals as subhuman - those that have done wrong have to be punished.
Most people see themselves as the heroes in their life story. When I harm Bob, and Bob escalates, and harms me back, I won't see it as just desserts - I see it as Bob harming me, context be damned. I will retaliate. (And so will he.)
Obviously, murder is a very serious form of retaliation, but with no path for de-escalation (a trial, a judgement by a religious, or community elder - or a feudal warlord), eye-for-an-eye will eventually reach that point. Let's not forget that most cultures, including ours, prefer, or at least tolerate revenge over turn-the-other-cheek. It's why it's so easy to treat criminals as subhuman - those that have done wrong have to be punished.
Most people see themselves as the heroes in their life story. When I harm Bob, and Bob escalates, and harms me back, I won't see it as just desserts - I see it as Bob harming me, context be damned. I will retaliate. (And so will he.)