I hope he's just doing it for political effect. The death penalty is under attack. We don't need the staunchest conservatives turning against it.
Of course, I don't have any problem with tightening it up to make sure that innocent people don't accidentally get executed. That's the last thing we want to happen, particularly since it would be very difficult to stop repeal of the death penalty if that happened. But the notion that we should simply take some classes of criminals who are clearly guilty and exempt them from the death penalty does not appeal to me.
On a moral level I have no problem with the Death Penalty. The problem is that our justice system is too coarse/crude/inept to be trusted with such an absolute penalty.
Even now prosecutors around the country are fighting efforts to use post-conviction DNA testing to exonerate innocents.