To be guilty of 1st degree murder there must be an actus reus (a guilty act) and a mens rea (a guilty mind). If the metal capacity was not there to reach the standard of the mens rea, then they should be aquitted. Society, as determined from case law and statute, prefers to treat mentally ill people rather than punish them.
Are you happy to execute people who are mentally incapable of controlling their conduct?
Since you to continue to misrepresent and make straw men despite my very clear words, I have nothing more to say. If you wish to engage with what I have actually said, then I will respond but it seems more like you have a preformed conclusion and this is not conducive to debate.