According to the new interpretation of the Bill of Rights approved by five Supreme Court justices this week, if an al Qaeda terrorist who was 17-years-and-364-days old detonated a dirty bomb in a U.S. city, murdering hundreds of thousands of Americans, no state could execute him. That is because executing any killer under 18--no matter how cold-blooded his crime--would violate America's "evolving standards of decency" and thus is prohibited by the 8th Amendment, which bars "cruel and unusual punishment." So said Justice Anthony Kennedy in Roper v. Simmons, an opinion that Justices John Paul Stevens and David Souter joined, and...