The death penalty is *not* punishment, since it is impossible for the recipient to “learn” from it. It is purely retribution and elimination of an undesirable component of society. The only function a death penalty recipient serves is as an example to others. Therefore the actual recipient’s intelligence (or lack thereof) has no bearing on the effectiveness of the sentence *on the recipient*. In other words, no one is too dumb to die for a crime they committed.
Really? No one is too dumb to die for a crime they committed? That doesn't seem very likely. There are people that don't have the cognitive ability to understand how to dress themselves, or feed themselves who could conceivably murder someone. Should they be put to death?
It's seems to me that in a civil society, there must be at least some minimum capacity on the part of the defendant to understand the criminality of their actions. To be clear, an absence of cognizance ability to understand criminality doesn't equate to ignorance, which is no excuse under current law. As an example, I might not know that the speed limit is 65, but that doesn't absolve me of the ticket that I receive when driving 70. But, if I can't understand the concept of speeding or right & wrong and certainly murder, then it's tough to make a compelling argument for the death penalty in such instances.
I have no idea if this man is competent to understand his crime, but there certainly are people who lack the mental acuity to understand concepts as simple as death, or murder. I think it ignorant to pretend otherwise.
One other purpose: it eliminates recidivism in the particular instance.