I don't think they should be executed, but they should have the hardest time possible...perhaps when found to be guilty they should be sent to a tougher facility, or given different uniform in prison so that they stick out like sore thumbs for the other inmates to identify easily.
Alternatively, we could do a deal with countries with far rougher prison systems (see Russia/most of South America) and do an exchange; their most well-behaved prisoners for our terrorists.
They will be left there to rot, open to ridicule by both inmates and any wardens that may or may not be there given the level of racism known in both areas.