The thing I'm arguing for is banning instances that open a floodgate.
So using your Johnny Regular example, if he goes postal, they can handle Johnny Regular, not our problem, they had no way to know
But if the server admins are like "🎉 Hey everyone! I know that you will all love to see him, please welcome Mass Murderer Johnny Regular! 🎉", that's when you ban the entire instance.
The SS was a government organisation, are you saying we should welcome them into our servers? Islamic State was a government organisation, are you saying we should welcome them? - No, we shouldn't.
We're not legalists, just cause it's legal and officially supported by a government doesn't magically make it moral. If an org does terrible things, and that's well known, then allowing them to join is always bad. It's implicit support of them.