In 1992, before delivering the Supreme Court's ruling in an abortion case, Justice Anthony Kennedy stood with a journalist observing rival groups of demonstrators and mused: "Sometimes you don't know if you're Caesar about to cross the Rubicon or Captain Queeg cutting your own tow line." Or perhaps you are a would-be legislator, a dilettante sociologist and a free-lance moralist, disguised as a judge. Last Tuesday Kennedy played those three roles when, in yet another 5-4 decision, the court declared it unconstitutional to execute people who committed murder when they were under 18 years old. Such executions, it said, violate...