They have repeatedly told anyone within earshod at Gitmo that
they will kill Americans and are simply waiting for the chance.
They have also made more than a couple attempts against the lives of the Soldiers who are guarding them. When
they were captured,
they were attempting to kill Americans.
The problem with your argument is the use of "they." Surely almost all of these people are very very dangerous ones, but some of them may not be. Some may have spared their lives by giving up intell. To say lets just wipe 'um all out and be done with it imho is an overreach.