From the Internet Modern History Sourcebook:
In 1936, Stalin began to attack his political opponents in a series of" purges" aimed at destroying the vestiges of political opposition to him. What follows is the official explanation from textbooks published before Stalin's excesses were repudiated by his successors.
The achievements of Socialism in our country were a cause of rejoicing not only to the Party, and not only to the workers and collective farmers, but also to our Soviet intelligentsia, and to all honest citizens of the Soviet Union.
But they were no cause of rejoicing to the remnants of the defeated exploiting classes; on the contrary, they only enraged them the more as time went on.
They infuriated the lickspittles of the defeated classes - the puny remnants of the following of Bukharin and Trotsky.
These gentry were guided in their evaluation of the achievements of the workers and collective farmers not by the interests of the people, who applauded every such achievement, but by the interests of their own wretched and putrid faction, which had lost all contact with the realities of life. Since the achievements of Socialism in our country meant the victory of the policy of the Party and the utter bankruptcy of their own policy, these gentry, instead of admitting the obvious facts and joining the common cause, began to revenge themselves on the Party and the people for their own failure, for their own bankruptcy; they began to resort to foul play and sabotage against the cause of the workers and collective farmers, to blow up pits, set fire to factories, and commit acts of wrecking in collective and state farms, with the object of undoing the achievements of the workers and collective farmers and evoking popular discontent against the Soviet Government. And in order, while doing so, to shield their puny group from exposure and destruction, they simulated loyalty to the Party, fawned upon it, eulogized it, cringed before it more and more, while in reality continuing their underhand, subversive activities against the workers and peasants…
The investigation established that in 1933 and 1934 an underground counter-revolutionary terrorist group had been formed in Leningrad consisting of former members of the Zinoviev opposition and headed by a so-called "Leningrad Centre." The purpose of this group was to murder leaders of the Communist Party. S. M. Kirov was chosen as the first victim. The testimony of the members of this counter-revolutionary group showed that they were connected with representatives of foreign capitalist states and were receiving funds from them.
The exposed members of this organization were sentenced by the Military Collegium of the Supreme Soviet of the U.S.S.R. to the supreme penalty - to be shot.
Soon afterwards the existence of an underground counter-revolutionary organization called the "Moscow Centre" was discovered. The preliminary investigation and the trial revealed the villainous part played by Zinoviev, Kamenev, Yevdokimo and other leaders of this organization in cultivating the terrorist mentality among their followers, and in plotting the murder of members of the Party Central Committee and of the Soviet Government…
The chief instigator and ringleader of this gang of assassins and spies was Judas Trotsky. Trotsky's assistants and agents in carrying out his counter-revolutionary instructions were Zinoviev, Kamenev and their Trotskyite underlings. They were preparing to bring about the defeat of the U.S.S.R. in the event of attack by imperialist countries; they had become defeatists with regard to the workers' and peasants' state; they had become despicable tools and agents of the German and Japanese fascists.
The main lesson which the Party organizations had to draw from the trials of the persons implicated in the foul murder of S. M. Kirov was that they must put an end to their own political blindness and political heedlessness, and must increase their vigilance and the vigilance of all Party members....
Purging and consolidating its ranks, destroying the enemies of the Party and relentlessly combating distortions of the Party line, the Bolshevik Party rallied closer than ever around its Central Committee, under whose leadership the Party and the Soviet land now passed to a new stage - the completion of the construction of a classless, Socialist society.
From History of the Communist Party of the Soviet Union (Bolsheviks): Short Course (Moscow, 1948), pp.324-327, 329.