I'm not sure what you mean by the "without due process" part, but I think assassinating people is a legitimate option in situations where if it wasn't available you'd have to go to war. If you can achieve a political change by killing one person (the head), thats by far a better option than starting a war that would kill and injure orders of magnitude more.
Without getting into the death penalty issue itself, I don't think assignation is a punishment, its a tool for change. Heads of state are difficult people to get access to, you cant just walk in and arrest them. Dropping a bomb on them though, or slipping some poison in food, more dooable. If that helps you avoid a bloody war, the math makes it hard to say no.
Thinking about the "due process" part, I would say a (maybe secret?) declaration of war would be sufficient due process. The people we kill in war don't get trials before we shoot them, but its not said that there was a violation of due process. Its not that they're guilty or innocent, its that they were in the way of the social change we were trying to effect.