It's such a broad concept, just like plenty of ethical truisms it can serve multiple ends. I am sympathetic to the people in here saying it's an idea ghouls hide behind to avoid justice but come on, back and forth family blood feuds lasting multiple generations is all about revenge and it ruins the lives of those involved. Don't get bogged down by the mental loop of trying to uncover some moral system that has never been used by cynical status quo supporters, killing and violence have been tragic necessities for upending oppressive systems but like others have said, it should be about justice and not about vengeance at the core.