Under normal circumstances, I'd agree with that.
But in this instance, killing those people literally undoes the deaths THEY were responsible for. It's less about murdering his enemies than it is about UNmurdering his people.
Normally, one of the main arguments against vengeance is "There's no point to it, it's not going to bring them back." But in this case, it would LITERALLY bring them back.
It basically becomes a trolly problem. Is it okay to kill a hundred murderers to save one innocent person? How many Nazis are you willing to kill to undo the holocaust? And so on.
I don't think that's true. Given that he seems to get genuinely furious any time anyone badmouths Rimuru.