But Putin also need to beware. Dictatorships fall not only when they have implacable opponents but also exemplary victims: Steve Biko in South Africa, Benigno Aquino in the Philippines, Jerzy Popieluszko in Poland. Through their deaths, they awakened the living to the conviction that it was the regime that should die instead.

Today, Nemtsov continues to haunt the Kremlin. So do Sergei Magnitsky, Natalia Estemirova, Alexander Litvinenko and Anna Politkovskaya, to name just a few of the regime’s murdered adversaries. At some point, a growing list of victims will start to weigh heavily against Putin’s chances of staying in power. The death of a galvanizing opposition figure could be the tipping point.

Especially when the political-survival formula that has worked for Putin so far is coming unstuck. That formula — enrich your cronies, terrify your foes, placate the urban bourgeoisie with a decent standard of living, and propagandize everyone else with heavy doses of xenophobic nationalism — no longer works so well in an era of Magnitsky sanctions, international ostracism, a persistently stagnant economy, middling oil prices, unpopular pension reforms, and dubious foreign adventures.