Why is their mental state relevant to their crime? If, in a trial, it can be proved that the defendant was mentally disabled/insane to the point where they had no control of their actions whatsoever then fair enough, but if the defendant is found guilty of murder despite their mental state I don't think more sympatyhy should be given simply because they're deemed mentally disabled. I can't say I agree with the death penalty, but considering the US's incarceration rates they need to get rid of criminals in order to make space for new ones. They could probably reduce the rate by stopping the war on drugs but that'll happen when hell freezes over.