it really shouldn't have anything to do with an individual being an American or not. Hit list should not exist. Rules of engagement indicate that an individual cannot be engaged until until they are the aggressor. Conspiring to commit terrorism does not give authority for anyone to engage them.
Perhaps this is a bad analogy, but this is how I see it. If you witness an individual paying a hit man to kill someone, you do not- nor does any government agency - have authority to use lethal force to bring them to justice.