There is a contradiction if the distinction isn't made between the death penalty as a form of punishment or as a necessary means of protecting society. The former, is indefensible if juxtaposed with the belief in a right to life and the latter, in any society capable of producing prisons keeping inmates in near complete isolation, is nearly never the case.
The reason killing as a form of punishment is not justified by one who adopts the 'right to life' philosophy is that it doesn't protect any other peoples lives. The personalistic norm of John Paul II says a person is an entity towards which the only proper way to relate is love. The notion of 'doing unto others as they would do unto you' could be used to justify the death penalty is thrown out the door when you consider the personalistic norm, as do many prolifers. The personalistic norm doesn't mean surrender yourself or your family to invaders who want to kill you, when any notion of 'proper relations' has gone out the door.
The reason killing as a form of punishment is accepted as a plank of the Republican Party is that it is a tough way to deal with crime. That's not very philosophical or wise.
People often conflate these two very different ideas, but I'd like to hope we're moving away from the death penalty (and life sentences) altogether.