I don't really see how that's enforceable. I mean, how do you know? I could be president and have some dude go and kill Imanutjob and then lie about the whole thing. Sort of like Clinton.
In a sane world -- the President shouldn't have to order the assassination of a Castro, Chavez, Saddam, al Sadr, Imanutjob or similar miscreants....
A very senior member of staff should KNOW the nation/world would be better served with certain asshats -- dead.
Semper Fi