For Japanese macaque monkeys, status is important. If a high-status macaque bites a low-status macaque, the low-status monkey will not attack back. The risk is too great. But stick around and you might observe something interesting. Every once in awhile, one of those bullied, low-status macaques finds a weaker relative of that high-status bully. It doesn't always happen five minutes later, an hour later, or even a day later. But once they find the bully's weaker relative, they will attack. And they won't embark on this attack late at night, when nobody is watching. The attack will take place right in front of that same high-status macaque who once hurt them. It's as if that low-status macaque is saying, "Listen, I know you are stronger than me, have more allies than me, and could squash me anytime you want, but I know how to hurt you so its best that you leave me alone in the future."
Revenge. It is not the province of evil doers, it is how creatures maintain order in a social system.