There is not need to explain how the death penalty would have deterred Klebold. The death penalty is not for the purposes of a deterrent. It is for the purpose of removing deadly dangers to the people of a state.
The rationale, pretty ancient I might add, is concurrent and the same as the right to use deadly force to protect yourself from clear and present human dangers to you and your family.
Since the state is the people of the state, it acquires either exclusive or concurrents right that people have. Very old doctrine.
The Supremem Court is dead wrong, as they have been being dead wrong almost sequentially these days. An under 18 functioning sociopath is as deadly a danger to the people of a state as an over 18 functioning sociopath.
You have to remember that rarely, in the stack of criminal cases, is the death penalty sought and as often imposed. Klebold would have and should have recieved the death penalty.
There are certain twists of human character that a few years in jail, at terrific expense to those same people the state is trying to protect, will not cure. 