The official order was "capture", but the real order was probably "kill on sight".
As already stated, a kill avoids all the subsequent legal BS. No trial necessary for the biggest bad guy in the world. Just pop him and be done with it. Plus, imagine the Islamic extremist fury and activity at seeing their beloved leader paraded around the United States as a prisoner. Many deaths would probably result from the demonstrations alone. A clean and simple kill would be way less likely to stir up that hornet's nest.
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