It is an executive order. (Executive Order 12333, section 2.11) Saddam and his ilk are no longer in power and are not under the protection of this executive order. Moreover, they resisted being taken into custody and in turn were killed.
DoD Directive 5240.1 - DoD Intelligence Activities, April 25, 1988
4.4. Under no circumstances shall any DoD employee engage in, or conspire to engage in, assassination.
Executive Order 12333 -- United States intelligence activities, Dec. 4, 1981
2.11 Prohibition on Assassination. No person employed by or acting on behalf of the United States Government shall engage in, or conspire to engage in, assassination.
2.12 Indirect Participation. No agency of the Intelligence Community shall participate in or request any person to undertake activities forbidden by this Order.
Executive Order 11905 -- United States foreign intelligence activities, February 18, 1976
(g) Prohibition of Assassination. No employee of the United States Government shall engage in, or conspire to engage in, political assassination.
The full text of E.O. 12333 can be read at the following URL:
http://www.au.af.mil/au/awc/awcgate/...se/eo12333.htm