This is a dilemma. There are circumstances where I would most definitely order the hit of a U.S. Citizen.
If that person was off our shores on a large boat and was about to fire off a very strong WMD cruise missile at NY or another major or strategically important target, I'd give it about half a second's thought and sign off on it.
Get him. Get him NOW!
The TARGET would not the be American, the TARGET would be the WMD. Problem solved.