A question on capital punishment

This question has bothered me for awhile (most recently in the "Nazi scientists [brought] to the US" thread) since I'm apparently missing some argument that some people apparently feel is compelling: Why is it considered less morally objectionable to execute someone who is intelligent than it is to execute a moron?

Comparing retarded adults to children is flawed because children can grow up and contribute something to society. Intelligent people can also (theoretically at least) pay part of their debt to society as well. If there is a death penalty anyway, shouldn't there be some calculation like (potential contribution to society) - (potential threat to society) > 0 or something?

/Something is wrong when people try to feign stupidity or craziness to lessen their punishment. Let them pretend to be smart and coherent instead.