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.
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.