Of all the reasons to oppose the death penalty for Dzhokhar Tsarnaev if he is found guilty of the Boston Marathon terror bombings, none is less convincing than the claim that executing the murderer of Krystle Campbell, Lingzi Lu, Martin Richard, and Sean Collier would amount to rewarding him. Seek Tsarnaev's death and you give him just what he craves, runs this argument, which has been made by law professor Alan Dershowitz, criminologist James Alan Fox, and death-penalty critics in the media. Executing Tsarnaev will succeed only in making him a martyr and a hero, they say; it will drive...