Sergei Kirov was assassinated on December 1, 1934 in the corridors of the Smolny by Leonid Nikolaev, a party member and former employee of the Leningrad Oblast Committee. Nikolaev immediately attempted to kill himself but was foiled by several Smolny staff members who had heard the murderous shot and came running, including Mikhail Borisov, Kirov's bodyguard.

Unlike other would-be assassins, it is clear from primary documents that Nikolaev was mentally unstable and having problems in his marriage to Milda Draule, an attractive young secretary who was rumored to be having an affair with Kirov (though no evidence of this exists).

Despite this fact, the event was exploited for political purposes. Soviet newspapers reported the murder as a political assassination and that Nikolaev was part of a Zinovievist opposition group. In order to link the two, evidence was manufactured and Stalin personally oversaw the orchestration which led to a series of trials in 1934-1935 that resulted in dozens of oppositionists being sentenced to death or imprisonment. Zinoviev and Kamenev were also put on trial based on fabricated evidence.

In short, the killing was a murder by a mentally unstable, possibly jealous husband, but was exploited by Stalin for political purposes to persecute his opponents.

There have been many attempts to link Stalin to the murder, but there has never been any proof, and the motive typically offered - Kirov's growing popularity and possible opposition to Stalin - doesn't have any basis in reality. Kirov was a faithful Stalinist, a devoted bureaucrat to Stalin's order.

Source: Khlevniuk, Oleg. Stalin, New Biography of a Dictator