I am pro-DP, but many things must be taken into consideration before I would agree to execute the execution. The suspect may have already spent time in prison. This may result in not only the patient not getting the counseling he/she needs, it also may have subjected them to be influenced by very bad people.
Other facets would have to be examined too, was the person exposed to a living hell by parents who did all kinds of heinous acts against them for years? Did this negative mental conditioning influence the action of the suspect?
As NTG pointed out this is a touchy subject. If we keep them alive to study them, what kind of experiments would be deemed humane? This also presents the problem of possibly mislabeling someone a killer before they have actually done anything wrong. How can we be sure they will carry out their threat? Many suicidal people may consider and threaten to end their own life, but never go through with it. Would we put someone in prison for the rest of his or her life because they show the traits similar to previous serial killer?
One thing is for sure though, neither the mentally ill or hardened criminals are getting the rehabilitation they need in our prison system.