If you want to know about Stalin I think the best source is Leon Trotsky:
Stalin knew that having betrayed the Revolution, he needed to eliminate those who defended and embodied the ideas of Bolshevism and world revolution. First and foremost this fell to Leon Trotsky, who had been driven into exile some eleven years earlier. All the resources of the Russian state were now set in motion to carry out his assassination. Before long, Stalin had murdered several of Trotsky’s co-workers, seven of his secretaries, and four of his children – the latest being his son Leon Sedov in early 1938.
Trotsky was an outstanding revolutionary and theoretician. As early as 1904, he expounded this theory of the Permanent Revolution, which was confirmed in practice by the October Revolution. Trotsky had spent his entire life in the revolutionary movement, led the Petrograd Soviet in 1905, led with Lenin the October Revolution in 1917, created the Red Army from scratch, helped build the Third International, then, fighting against the Stalinist bureaucracy, was cast from power and forced into exile by Stalin.
in January 1937, with the announcement of Stalinist trials of Radek, Pyatakov and others, Trotsky literally put his life on the line to speak the truth about a Revolution betrayed: “How could these Old Bolsheviks, who went through the jails and exiles of Tsarism, who were the heroes of the civil war, the leaders of industry, the builders of the party, diplomats, turn out at the moment of the ‘the complete victory of socialism’ to be saboteurs, allies of fascism, organisers of espionage, agents of capitalist restoration? Who can believe such accusations? How can anyone be made to believe them? And why is Stalin compelled to tie up the fate of his personal rule with these monstrous, impossible, nightmarish juridical trials?
“First and foremost, I must affirm the conclusion I had previously drawn that the top rulers feel themselves more and more shaky. The degree of repression is always in proportion to the magnitude of the danger. The omnipotence of the Soviet bureaucracy, its privileges, its lavish mode of life, are not cloaked by any tradition, any ideology, any legal forms. The Soviet bureaucracy is a caste of upstarts trembling for their power, for their revenues, standing in fear of the masses, and ready to punish by fire and sword not only every attempt upon their rights but even the slightest doubt of their infallibility. Stalin is the embodiment of these feelings and moods of the ruling caste: therein lies his strength and his weakness.”
(Writings, 1936-37, p.121)
Powerful stuff I think you'll agree?
He went on to describe the Moscow frame-ups as “the greatest political crime of our epoch and, perhaps, of all epochs.”
The Revolution Betrayed