Tis a slippery slope, Mr Holder...

The Attorney General believes that it is legal to... how did he put it...  use "Targeted Killings" against US citizens who are suspected of plotting to kill Americans. 
"Let me be clear: An operation using lethal force in a foreign country, targeted against a U.S. citizen who is a senior operational leader of al Qaeda or associated force, and who is actively engaged in planning to kill Americans, would be lawful," he said.
He even states that there are criteria that must be fulfilled:
The U.S. government must have determined that the individual poses an imminent threat of violent attack against America; capture of the suspect is not feasible; and the operation would be conducted within the principles of the law of war.
Holder argued that al Qaeda has the ability to spring surprise attacks and is considered to be continuously planning against to attack on America. Therefore, the law allows for striking even before the "precise time, place, and manner of an attack becomes clear."
The issue here is that the person in question is only suspected of planning.  Not proved to be in a court of law, just suspected to be.  No due process, no trial.  If they think someone is planning an attack and is not on US soil, the can assasin...  Oh wait, use a targeted kill on the suspect.

Lets examine some of those words:
Assassinate - According to Mirriam-Webster, the second definition is "to murder (a usually prominent person) by a sudden and/or secret attack, often for political reasons."
Due Process - Due process is the legal requirement that the state must respect all of the legal rights that are owed to a person.

Based on what I read, Targeted Killing appears to be suspiciously like assassination.  And since the people in question would be US citizens, this seems to violate the Due Process guaranteed by the Fifth Amendment quite clearly.  Additionally, since we are not at war with an actual country at this time, any targeted killings of US citizens outside the US using the "principles of war" would be suspect at best.  Top this with the "I know they will commit a crime, just not where, when or how" aspect and we have a recipe for disaster. 

To the best of my knowledge, US citizens do not give up their right the moment they leave US soil.  With the ambiguous criteria laid out by the AG, in addition to no judicial/grand jury oversight, this starts us down the slippery slope mentioned in the title.  Since this is the same AG that brought us Fast and Furious as well as deciding not to prosecute the Black Panthers for voter intimidation, I am a uneasy that he would be in charge of making policy like this.  Actually, it scares the hell out of me.  When you combine this with the head of Homeland Security's decision making, I truly fear for our citizens.