Nemtsov was the only Russian opposition leader who had held both a major elected public office, as governor of the Nizhny Novgorod region (appointed 1991, elected 1995, resigned 1997) and a senior government job, as vice PM in reformist Yeltsin cabinets (1997-8). He was one of the leaders of the still-influential Union of Right Forces in the Third Duma in 2000-4.
I cannot recall an earlier murder in Russia of a public politician of comparable stature except, perhaps, that of the popular Duma deputy Galina Starovoytova in St. Petersburg in 1998. Also, Sergei Yushenkov, a prominent Union of Right Forces parliamentarian, was shot in 2003.
The site of the killing would normally be considered one of the safest in Moscow, closely watched by police and security services of all kinds. Recall how fast they reacted when opposition activists tried to hoist a Ukrainian flag upon a nearby bridge.
The Kremlin will likely make a major effort to pin Nemtsov’s murder on the Russian opposition and/or the Ukrainians, and use it as a pretext for the persecution and demonization of Russian dissidents.
Murder, opportunity and motive [updated March 1]: Judging by the location of the murder and by the way things work in Moscow, however perfunctory my understanding may be, it’s uncomfortably clear who had the most fitting means and the greatest opportunity for committing the crime. There is far less clarity about the motive, but for the time being, let us simply note it is not unusual for dictatorships to murder political opponents.