They might want to check out this story -- of a woman who was relentlessly stalked and then murdered by a man who spared no trouble researching the implications of his crime:
[The country prosecutor] said [the self-confessed murderer] had done research on the Internet to determine if Illinois had the death penalty, deciding to go through with [the victim]'s murder when he discovered it does not.
The next time someone tries to claim that the death penalty makes no difference when it comes to saving the lives of innocent victims, think of the well-known story about the starfish, think about the victim in this case, and point out that "it could have made a difference to this one."