The Supreme Court's death penalty jurisprudence is unprincipled. Capital punishment certainly was not considered "cruel and unusual" at the time the Constitution was adopted, or for nearly two centuries thereafter. So what the Court has done in recent decades is to make it up as it goes along. Today it held that while it is constitutional for a state to punish murder by death, it is prohibited to punish child rape--in the case at issue, the rape of an eight year old girl by her stepfather--by death. Needless to say, no such distinction appears in the Constitution. It is solely the product of arrogant quasi-politicians who have been in Washington too long.
Even Barack Obama couldn't stomach this one, in an election year. But that's only a ruse: bad as Republican appointees have often been, any Justice appointed by Obama would be selected precisely for his eagerness to impose his liberal views on "unenlightened" voters.