Obviously, public hanging would be a pretty irresponsible thing to do, but they definitely should get death, if for no other reason than to negate the possibility of giving them back.
Another point: Why is everything negotiable? Certain things are supposed to be off the table. They should be able to announce before starting any prisoner swap that they will absolutely not release a cold blooded murderer. The same goes for land: certain cites are out of the question.