I am afraid you are mistaken in your estimation here, Kacey.
If you are aware of any studies that support a relationship between capital punishment and criminal deterrence, then feel free to post them. But, until then, the Burden of Proof is on those that posit there is such a relationship. Such a burden is not on those maintaining no evidence for a relationship presently exists.
Furthermore, this still ignores the fact that the specific claim Ceicei gave is probably untestable to begin with. How, exactly, does one demonstrate that capital punishment is a deterrent to serial killers?? How would you even begin to approach testing such a hypothesis??
My point precisely - under current conditions, it is not possible to test this hypothesis either way. That does not make it proven or unproven; it makes it unproveable. What
has been proven is that the current system is not working - of we wouldn't be having this much trouble with recidivism, or with the crime rate in general.
Capital punishment has been around a long time - not beginning with the Law of Hammurabi, which called for the punishment to fit the crime. At the same time, modifying the punishment based on the relative status of those involved has also been around a long time - on the Old Testament, for example, the value of a transgression was lighter if a freeman transgressed against a slave.
You are welcome to your opinion - which, as things stand, is no more proveable than mine or any other. This is a complex social issue, and no study of a single facet will determine the correct answer - indeed, there are, quite likely, multiple approaches that could prove effective. However, I am no more willing to accept your unsupported opinion than you are mine, and that is my option. If you have something to say other than "I'm right, because your opinion is unproveable" - especially when your position is equally unproveable - then, please, step forward. If not, well... enjoy yourself, but don't expect to be taken seriously; it's a weak argument, at best.