The US should only kill an American citizen when he poses an immediate threat of deadly harm and there is no other way to stop him. I am not sure that these conditions were met in the recent assassinations of American citizens Anwar al-Awlaki and Samir Khan.
According to the press, Awlaki encouraged other Americans to kill their fellow citizens and to oppose the US government, but it's not clear that he personally killed any Americans, or anybody else, for that matter. He was more an accessory to murder than a murderer. Secondly, its not clear that there was no other way to stop him than to kill him by remote control drone. That may have been the easiest way to kill him, but not the only way.
I think there should at least have been an effort to take him prisoner and return him to the US. I also think we should have tried to capture and return Osama bin Laden. The problem is that the US legal system is unable to deal with terrorists, because Americans are so afraid of them. Guantanamo should have been closed years ago, but Americans are afraid of the men there. There was some talk of a terrorist trial in Kentucky, and Sen. Mitch McConnell almost had a fit he was so scared. This is a man who refused to fight in Vietnam, and got his patron, Sen. John Sherman Cooper, to help get him out of military service during the war, although officially he got a medical discharge.
These legal niceties are what our troops are supposed to be fighting to protect, but we are afraid to apply them. In many ways Osama bin Laden won, because people like Barak Obama and Mitch McConnell are afraid to stand up for them. Of course, the real cowards were George W. Bush, who spent the Vietnam War becoming a drunkard in the Alabama National Guard, and Dick Cheney, who avoided service by churning out babies. These are men who liked running the country, but had no concept of what it was to serve the country. They were missing in action on 9/11. Bush flew away to Nebraska or somewhere, and Cheney retreated to a spider hole under the White House.