Well now, you are splitting hairs aren't you? No one is talking about random killing, are they? Let's be sure, of course. Take all the time necessary to get required information.
Until a couple of years ago, there was a pretty good plan for prosecuting terrorists and those pesky non-state combatants using military tribunals. And it would not be too much of a stretch to impose mandatory life or death sentences upon conviction.
I am inclined to go the military tribunal route for all those foreign terrorists who get so far as to be charged. Good lawyers like Tom Cruise and Kevin Bacon and, my personal favorite, Catherine Bell, can square off. The revolution will not be televised, folks, but court records will be kept.
Americans citizens get to choose Door #2 and due process in a special federal civilian court designated for crimes of mass destruction, assassinations, etc., all standard protections apply. However, no Mirandizing until they get "debriefed" to the fullest extent possible. Take all the time you need.
If you renounced your American citizenship in pursuing this new career, or went overseas to kill, kill, kill, well, you get to go through Door #1 as I don't think you really qualify to be tried as an American.
Let's look at our little buddy Umar Farouk Abdulmutallab. Anyone doubt he tried to kill a couple of hundred people and was hoping for a few thousand more?
He gets wicked crotch burns for his effort and wins an extended session with professional interrogators! Once they get everything useful out of him, give him his Miranda warning, an ACLU lawyer, a trial (we won't use any involuntary testimony) and then a choice of life at the local SuperMax or a chance to show us how well he can fly from an airplace at 10,000 feet over Lake Michigan. Sorry Umar! All out of parachutes for convicted terrorists!
How about our fat buddy KSM? One bad boy there. And smart. Yessiree. I would be inclined to give him an extra special long time to tell us everything he knows. Then Door #1. The hard part will be deciding what reward to give him after the trial. Suggestions from victims' families are welcome.