I can't help but wonder if there are not other, possibly quite justified, interpretations of things like Anwar Al-Aulaqi than "execution without trial." I mean, was he not actively engaged in terrorist activities? Had he not been already convicted of other crimes, including by the government of Yemen? Thus, could he not be considered as much of a justifiable target as Osama bin Laden?
Think about this: would it be wrong to treat an American citizen who declared war on the US from some other nation as an armed enemy, and treat him according to the rules of war? Would it be technically required to NOT kill him, and instead spend billions trying to capture him and bring him home for a "fair trial?"
Among the primary duties of an American President, as I understand it, is the protection of America and its citizens.
Further, it was only really a year, maybe a year-and-a-half, into Obama's Presidency that drones became really useful, really technically controlable, in a way that Bush simply didn't have. To say that the side with the most guns did the most killing with guns doesn't make them any more guilty of killing than the side with only swords and bows and arrows, who killed as many as they could with those weapons.