I'm not sure which is a worse mindset or motivation, since it doesn't matter worth a damn in the end, but from what I have seen, I tend to think that Ulyanov and Bronstein were actually sincere in their commitment to building a better (in their minds) world, no matter how many eggshells they had to break. They were socialism and progressivism taken to the logical end. Dzhugashvilli, on the other hand, was just a thug gangster who was interested entirely in his own success and aggrandizement. He was much more practical in his application of the same atrocities and tactics, and I can't see him letting procedural and parlimentary tricks marginalize him like the Old Bolsheviks did.

It's kind of an unanswerable question about the worst thing about a totalitarian system: is it the inhumanities people can commit in the name of ideals unrestrained by tradition and respect for human dignity, or is that it allows someone like Stalin to hold power through the apparatus of the state, rather than merely filling in a void as gangsters do in natural societies.

The exasperating thing is that progressives try to act as if the body counts of these guys are an unfortunate side effect, or personality defect of the individuals in question, rather than the eventual outcome of progressivism taken to its logical conclusion, while at the same time demonizing religion and predicting or accusing it of basically aspiring to crimes only progressivism has managed to achieve. A favorite trope of speculative fiction writers is a tyrannical theocracy whereby people are punished for insufficient fervor, where reality and science is ignored in favor of doctrine, and blind zealots ignore the cost of their devout policies. And of course, allowing cynical brutes to carry out their pursuit of power and exercise their violent tendencies by merely using the terminology and aping the practices of the faith. And never in human history has organized religion done any of that on the scale that Stalin, Lenin, Trotsky and Lysinko pulled off.

It's noteworthy that when creating a setting of the ultimate repressive theocracy, the Warhammer 40K writers might have used religious terminology, but had to turn to communism for inspiration in the practical exercise of inhuman tyranny.