The proof of criminal intent is much more subjective. It involves determining if the defendant is capable of forming INTENT or RECKLESSNESS. If we concede that the crime was voluntary, then we rule out recklessness, which is passive. It only remains to prove intent.
The claim is advanced that retarded persons are incapable of forming criminal intent, since they are functionally unaware of the consequences of their actions, or that their actions may cause harm.
That then is the argument that protects retarded persons from the death penalty.
I believe that the victim of a retarded person is just as dead as the victim of a thoroughly competent murderer. I also believe that true justice demands retribution from the criminal. Therefore, I would disregard intent, and would apply the same penalties to the retarded person I would apply to the competent. I would sentence the murderer to death, and would execute him with no remorse.
The world does not need to coddle murderers, regardless of their mental innocence. I would apply the same strictures to children.
By the way, most states require that extreme aggravating circumstances be proven before the death penalty can be applied. That alone would protect a retarded person, unless the crime was committed repeatedly or wantonly, which would belie a defense of reduced capacity.