You cant do this "legally", for that you would have to assume that the terrorists were aware of the "laws" and were knowingly breaking them. These people dont have the same beliefs and values we do. I use the terms "we" and "these people" generically and dont mean to upset anyone.
As far as retaliation or revenge making us barbaric or savages, that book so many of you put so much faith in(the Bible) says an eye for eye. But remember, these people also believe they are acting on behalf of their God most of the time too.