[etlib]

Protagoras: I would never kill anyone because of my morality, but if I had no morality, I might kill you for disagreeing with me if I felt like it and thought I would suffer no ill consequences to myself.

Junior: But you will suffer consequences. Your victim's family might come after you or your family. Your society might decide that you're too dangerous to keep around (you might kill one of them next).

You would suffer consequences because you live in a society where the consequences exist. In some societies it is permissible to kill those with whom there is a disagreement. For example, the duel for honor.

[Junior]

True, but in those societies there are strong rules surrounding even that. In a duel, for instance, both individuals are equally armed, seconds are available to ensure the rules are followed, and there is a referee. Each participant also understands the results of the duel are final and there will be no retribution by relatives of the loser.

Vendettas are extremely disruptive of society, so society evolves rules to handle them. The duel (and the code duello) you mentioned above are one. Scandinavians developed the concept of weregeld where the killer pays the family of the deceased to head off any retribution. These rules and formalities are so important that crime families are known to use similar such to head off general blood baths between them.