@JLNobody,
I don't think we know that Stalin was an atheist (although he probably was, at the least fuctionally)--he did start his adult life in an Orthodox seminary. The official Soviet biography's claim about him rebelling against the established church and imperialism is not born out by the records, which simply note that he eventually failed to pay his tuition.
The problem i have with this view is that it is promoted vociferously by christians who attack atheism, rendering as Atheism, and equating it with religious confession. Stalin didn't have people killed because they refused to embrace atheism. He didn't go to war to promote atheism. The so-called martyrs of the Russian Orthodox church were priests and monks who supported Kolchak's White Army in the civil war with Trotsky's Red Army. They died for a political reason. Priests, monks and nuns who did not oppose the Boshevik state were left to their own devices. The problem was that Petr Alexeevitch, "Peter the Great," had made the church an organ of the state. All priests, monks and nuns were paid a salary by the state, and the churchs and monestaries and convents were supported by the state. The successful Bolshevik revolution withdrew that support, and the church in Russia simply starved to death.
Stalin's first big murderous crime was the deportation of the so-called Kulaks. Despite the hysterical claims of far too many people, somewhat less than 500,000 died--bad enough, certainly, but not the millions that some people allege. The Bolshevik revolution came in November, 1917 (October in the Julian calendar--hence, Red October), after the Russian revolution in February (March in our calendar). But the Bolsheviks did not immediately succeed to power, and the two other large revolutionary parties--the Social Revolutionaries and the Peasants' Party--had "redistributed" land to many peasants. Marx understood that peasants who had gotten land would now want the revolution to end. They may have been glad to overthrow the previous status quo, but they had created a new one which they wished to preserve. So Stalin created the legend of the Kulaks, deported millions, which killed hundreds of thousands, and collectivized agriculture--in the process killing Russian agriculture. Not that he cared about that--preserving the ferment of revolution preserved his justification for his power. Atheism was no part of the equation, which was entirely political and politically ideological.
There we get down to the kernel of the phenomenon. Fanatics like that, absolutists if you prefer, use ideology, whether religious or political, to justify the program which puts them in power and preserves their power. Christianity spread quickly across Europe despite the contempt that many "pagans" had for it as petty rulers saw the value which an established church had for them. They protected and preserved the church, and the church povided the ideological rationale for their power.
So i'm not even sure that it's always a case of absolutists. The absolutists may be pawns as much as the oppressed peasant for whom the distinction between Tsar and Chairman of the Central Committee is meaningless. It can just as easily be a case of expedience in the service of personal ambition.