[Just another Joe]

Is it barbarian to injure, or kill an avowed terrorist, or allow an innocent to have his life taken ?

Injure or kill, in the heat of the chase, or during a firefight trying to capture the terrorist? No.
Torture? That just brings the US down to the terrorists level and gives them a justification for hating and wanting to hurt any US citizen. I know that a terrorist doen't need any justification but it could justify the terrorist in the eyes of the rest of the civilized world.

[hobbes1]

Perhaps, but by their actions they are already willing to take your life, hating you more is of no consequence.

Giving them pause to think twice is.