Apparently there is a statute that allows citizenship to be stripped. This is what I found so far:
"A federal statute says that a citizen loses his citizenship by "serving in the armed forces of a foreign state if such armed forces are engaged in hostilities against the United States" but only if he does so "with the intention of relinquishing United States [citizenship].""
So apparently serving against us must be accompanied by the intent to relinguish citizenship.
I don't know that this particular statute would apply given that the particular American citizen that has been targeted for assassination is not even suspected of serving in the armed forces of a "foreign state," let alone with an intent to extinguish his citizenship.
This policy sucked when Bush did it and sucks now.