I don't like that decision.
For one thing, you do the crime, you pay the price. And if it's decided that that price should be death, then that's what you get. I think that it's a rare situation where a so-called "retarded" criminal is SO retarded that he/she didn't know the difference between right & wrong.
It leaves all kinds of possibilities for people to "fake" being unintelligent. I could go commit a crime & say, "Sorry, I am stupid, I didn't know shooting someone could kill them." A low IQ is no excuse for murder. If someone with such a low IQ honestly doesn't know right from wrong, then what are they doing roaming the streets free, instead of with constant adult care or in a home?
I just don't like it at all. And no, I am not passionately pro-capital punishment, but if we are going to put killers to death, we should put them all to death....it's just too easy to claim you didn't know what you were doing.