A little more context would be helpful, but I suspect you understand “by what right” means. What gives one person or wild animal (sort of figuratively speaking since animals don’t think this way) doing something as bad as another the right to claim the moral high ground over that other person? One person is doing one bad thing doesn’t give that person doing a comparably bad thing the right to claim to be morally superior to another doing an equally bad thing. Without more, that’s about all it means. By what right does a thief judge an embezzler? They’re both stealing, so what’s the difference? As long as you’re talking about moral equivalencies, I think the question is appropriate.