He massacred a crowd of people who disagreed with him — that makes him leader of the swamp.
Justinian massacred a crowd of insurrectionist, murderous bureaucrats who wanted to maintain their lifestyle of parasitism and corruption and were pissy about his just reforms. They were demolishing the whole of Rome, and murdered thousands of innocent people, destroying most of the entire city, before he re-established order. It seems that those who hate Justinian do so because he did not count the lives of the murderous, seditious, greedy, parasitic bureaucrats so far above those of the tens of thousands of people who they senselessly slaughtered without any cause at all, other than that they stood between them and the target of their real hatred. And so satan’s aristocratic preachers like Gibbons aren’t so different than the swampdwellers they shed their acidic tears for.