There’s some interesting history here. The idea that you don’t kill leaders was pretty entrenched in Europe for the last few hundred years. Mostly, it was a gentleman’s agreement among monarchs to keep themselves safe. You might capture a king or queen, but you’d ransom them back to their nation rather than killing them. The French Revolution killed their king and this, more than anything else they did, made them pariahs on the international stage. No one cared that they killed large numbers of their own people (everyone did that, ask the Irish), but none of the other monarchs wanted the idea that you can just kill them to catch on.

After Napoleon called himself Emperor, he got the same protection. He wasn’t killed at the end of the war, he was exiled (and then again, further away, when the first time didn’t work). The leaders of the time thought killing him would set a precedent that they were uncomfortable with. There are even reports that he came within gun range during Waterloo but Wellington ordered the allies not to shoot him.

I’ve never been particularly happy with this convention because it exists to split the world cleanly into people who are responsible for wars and people who might die in wars.

I do find it particularly interesting that it’s the President who wants to be a King who is willing to upend this convention. Even Putin has shied away from having foreign leaders assassinated (though he has been very willing to have other people assassinated on foreign soil). He must have a lot of confidence in the Secret Service.