Killing a terrorist would be giving them the easy way out. Some terrorist believe that they will get a bunch of virgins for dying in a jihad,
Since we are on the topic of cruel punnishment, I will say this. Lets start with all the "torture" techniques that the US has now banned from use. If it doesn't kill the person, it is good to go. THen we should make them watch the great TV programs of the US: Barny, Martha Stewart, a few gay channels, watching the US senate debate any given bill, ect for days and days. That would be much more terrible than a bullet in the head.
Now since I am a realist, my version of punnishment is purely fiction. I have no clear answer for what we should do for terrorists. The easiest way to rid ourselves of terrorist is thru combat. No one can really defend the rights of a person once he/she is dead. Once captured, things get complicated. I think the best way to combat terrorism is to prove to the people that we are not the big bad US that everyone says we are. THat means spending a bit more on aid than we are doing right now.