We'll never stop people getting the idea. It would be a lot less appealing if they knew the rest of the world were going to have a serious and continual effort to kill the leader of the country that invaded until the war ended. Right now they basically get a free pass picking front lines far from their home.
Let's call the standard a single civilian death. This way you know which countries currently at war both get their leaders killed.
New leader after the assassination continues the war? Kill them too.
