No, offing them would not meet out needs. The reason they are being held.. alive.. at Gitmo is to try to break them down and get them to divulge important info about Al Quaeda or other terrorists still on the loose.
That is why we need to keep them in a controlled environment so that we can slowly (I'd prefer quickly but Human Rights Watch will have none of it) break these lowlife scum down and get some meaningul intel out of them.
Bush could always say: "OK, sure we'll put them on trial, but realize its gonna take us.. oh.. maybe another coupla years or so ... just to gather all the evidence we need!! We want to be fair, ya know and make sure we got the right guys!!" (LOL)
Not question who they were, or why they were here.
As best I can judge Gitmo is a lot more like the Cook County Jail: you have a few Sociopaths, you have some high and middle level hardcore gang members, you have a lot of lower echelon operatives, some people who are either players on the periphery of the gang or just wanna' bes, some mental ill indviduals, and even some ordinary if unsavory loking citizens who were caught up in a police sweep.