Without a definitive judicial ruling or other galvanizing event, a perennial American argument is ending. Capital punishment is withering away.

It is difficult to imagine moral reasoning that would support the conclusion that an injustice will be done when, years hence, the death penalty finally is administered to Dzhokhar Tsarnaev, the Boston Marathon terrorist who placed a bomb in a crowd and then strolled to safety. Sentencing to death those who commit heinous crimes satisfies a sense of moral proportionality. This is, however, purchased with disproportionate social costs, as Nebraska seems to be concluding.