I agree with you that people believe their death threats are justified, and probably aren't like "oh that's not me in real life."

But also, for me personally, I find it way easier to be an asshole online than in real life. I've definitely said things online (thrown insults, name-calling, etc) that I would never say in real life, and I would probably say, "that's not really who I am." But it is, because I said it. But it also isn't, because I was younger and dumber and more emotionally unregulated. Yes, it was still me who said it. But when you see someone's face, you see their expressions, their body language, you see them as a human rather than a username. And I understand for me, personally, that I'm more likely to be a dick behind a screen than to someone's face absolutely unconsciously, so I'll give someone else the benefit of the doubt there, as well.

In the end I would probably push back against the notion that we are always one person. We all should be capable of kindness and sternness, gentleness and seriousness, somberness and goofiness.