"The US Supreme Court has decided that, if you murder someone, you can still be executed, but if you're really, really stupid and you murder someone, you live."

Has to do with intent, understanding actions and consequences, basically. For instance murdering people with a car while drunk isn't a capital offence, as there is usually no intent to deprive people of life. (In this case, because of the possibility of repeat offences, I think it should be death penalty eligible.)

For actually retarded people, the court is saying there has to be a capacity to understand to some degree. Some people have never had the ability to understand killing a human being as much different than swatting a fly. For those without the intellectual understanding the death penalty may be "overkill".

And there are those people without the moral conscience to make that distinction. For them the death penalty is the only reasonable remedy.

"Doesn't quite compute for me."

Clearly, that's the intellectual and not moral type of not computing. You may be safe from the death penalty. Relax, :-) Just kidding, couldn't resist it.

Arf