When Vladimir Putin, Russia’s president, said that the assassination of Boris Nemtsov on February 27 was a “provocation,” it was natural to think he was implying that Putin’s own enemies were responsible. Such a proposition would fit with the Kremlin line that its critics aim to fracture society and overthrow Russia’s elected rulers. And the provocation formula might handily be extended to other enemies of the regime, at home and abroad. Sceptics, including many in the West, are understandably inclined to see a smokescreen, and to consider Putin directly or indirectly responsible for Nemtsov’s death. Either way, the killing may...