On Friday night's Real Time, Sullivan and Greenwald debated the assassination of Anwar Al-Awlaki and, naturally, Greenwald opposed the killing while Sullivan and Maher defended the killing.
You can watch the debate here.
As soon as someone takes up arms against the United States, they've entered a field of battle, and a war, of sorts, is engaged. Likewise, if a criminal aims a deadly weapon at a soldier or law enforcement officer, that criminal runs the risk of being fatally shot -- and it's perfectly legal to do so.
While that example isn't exactly a 1:1 analogy, since Al-Awlaki wasn't literally aiming a weapon at an American when he was killed, another point of comparison is the U.S. Civil War in which half the nation seceded and took up arms against the U.S. military and president. Should Lincoln, who considered the Confederates to still be Americans, have spared the 250,000 rebels who were killed during that war after they collectively threatened the stability of the United States? Clearly, and for a variety of reasons, the U.S. military effort during the Civil War, while brutal, was justified. Similarly, Sullivan brought up the American-joining-the-Nazi-Army concept.
I agree with Greenwald that due process and justice is crucial -- but only in criminal cases where the assailant isn't engaged in a shooting war against American citizens and soldiers.