Retaliation to protect oneself is not punishment because it need not be proportionate.

Evidence

If the wrongdoing is very serious and threatens the safety of the victim, she may retaliate in order to preserve her reputation or deter future aggression (McCullough et al. 2010). Such behavior, however, cannot be seen as punishment per se since it is aimed only at protecting the victim (as in many nonhuman species; Clutton-Brock & Parker 1995). Furthermore, although punishment commonly seeks to finely rebalance the interests of the wrongdoer and the victim, retaliation can be totally disproportionate and much worse than the original aggression (Daly & Wilson 1988). — Nicolas Baumard, A mutualistic approach to morality, 85

Categorization

• DEFINE: Offers a sharp definitional distinction: punishment = proportionate cost-setting that rebalances interests; retaliation = self-protection that may wildly exceed the original wrong.
• ADDENDUM: Ties into the sacrifice essay’s handling of disproportionate ritual violence — vengeance and sacrifice both exceed rational rebalancing.