I'll have to disagree with you there, SD. Truth is, criminals, as a whole, have limited ability to draw valid cause-effect relationships and have limited ability to delay self-gratification. While the DP might deter a robber from killing (that's not "his bag") it will likely have little effect on the rapist/murderer/child molester. Regardless, the main point is that the DP (in the instances of incorrigibly violent offenders) is not socially-preventative or punitive, it's individually-preventative.It certainly provides a deterrence. To think criminals are unconcerned with consequences or getting caught is naive.
The liberal/sociological argument against this is, "Well, if you're socially-cleansing by state-mandated executions, what is to prevent that from being applied to any minority that society finds offensive...?" And, the answer is: the Second, Fourth, and Fifth Amendments.
The sociological "problem" is that emphasis is placed on the rights of the deviant(s) rather than the rights of the citizen(used, since "citizen" implies a generally compliant, participatory, non-grossly deviant individual.) This is because sociologists view crime as individual response to social inequity, therefore the individual may not be held to the standards of the society since the standards are "unfair" (circular reasoning at its best.)