Ahmed Hijazi, a US citizen, was among six suspected al-Qaeda operatives killed by a Hellfire missile lauched from a CIA Predator drone in Yemen on 3 November 2002. To all appearances, he was, in fact, a member of a terrorist organization responsible for the mass murder of American citizens, and therefore at least as deserving of death as any other murderer. Traditionally, though, the final determination would be made by the judicial system rather than within an agency intended to inform and advise the President.
In response to questions raised by his case, the government has asserted the right to kill or capture anyone it suspects is an enemy combatant, anywhere in the world, at any time: even US citizens within the borders of the United States. (Jose Padilla's case raises this issue from another angle.) If you take this at face value, it means that the executive branch has the right to kill anyone and everyone it considers objectionable without so much as oversight by the judicial branch, just so long as it accuses the target of being an enemy of the state. In other words, due process is a mere formality that can be dispensed with at will.
The main target of the Hellfire attack was actually Qaed Senyan al-Harthi, who was also in the car and is believed to have been responsible for the October 2000 bombing of the USS Cole. "Ahmed Hijazi" is reportedly an alias for Kamal Derwish, alleged leader of the so-called "Lackawanna 6" terrorist cell, all of whom (other than Derwish) have been arrested and charged with two counts of providing material support to a foreign terrorist organization. Notice all the hedging and weasel words? That's why this is a problem: what little information is out there arrives via the executive branch itself.