Apologies because it's been some time since I've read the first two volumes of his trilogy so I can't give direct quotes, but Stephen Kotkin's biography on Stalin takes the view that his purges went back to the collectivisation efforts of the early-mid 1930s.

Essentially, his view, taking many speeches and the people involved into account, was that it was not simply the paranoia of a maniac or the result of a real, active plot to take Stalin down, but the result of party members, low and high, responding to the sheer scale of death and destruction of collectivisation.

During this state policy, the members of the Politburo discussed how best to implement this and when, but many disagreed with Stalin that it should take place now, arguing the country had still not recovered from the Civil War and this could ruin any progress in creating their Worker's Paradise, or even destroy the Soviet Union itself.

From memory, Kotkin notes this through-line in a speech given to the Red Army in the middle of the Great Purge- part-way through the speech, Stalin suddenly shouts "Collectivisation!" and begins talking about attempts to "wreck" these plans that had been implemented. These "wreckers", by and large, were simply people who were those as described above, wanting to implement it but thinking it would ultimately be dangerous in the present.

In one of Kotkin's many YouTube videos discussing the book at keynotes, he describes a conversation between Stalin with his staff which describes what he believes his thinking was (paraphrased): "Are we not Communists?- Yes.- Do we believe in Communism?- Also yes.- So why do you not want to go through with Collectivisation? Because people will die? Because damage will be done? History will judge us on this issue. This is something we must do." This is not a direct quote, again, but sums up his interpretation of Stalin's goals and motivations for the ordeal.

Put simply, Kotkin believes Stalin's motivation was not paranoia, not insanity, not a real plot, but Stalin's view that his country's government was filled with people who were not ready to actually put their money where their mouth is regarding their convictions, that they were not the people they said they were, that they were not really Communists who had the heart and the ability to implement Communism.

Now, I think with the massive (and I do mean massive, it's a gigantic tome of a series but reads very well) piles of evidence he gives that this view is accurate, but as with all views on Stalin, has many issues. For one, I just personally struggle to reconcile that someone would take what seems like relatively minor misgivings about a plan they would like to implement eventually and apply it as meaning that hundreds of thousands of people must be shot, tortured, imprisoned or enslaved, many picked personally by Stalin for seemingly no reason. But this also argues for itself- what better way to radicalise people into "having the convictions of their beliefs" than by getting them to eliminate and rat out the soft-hearted?

Seeing as this is Stalin, there will always be a thousand questions with a thousand answers, but Kotkin makes a compelling case for his view and reminds us that Stalin was at the end of the day a human being with human senses and human emotions applied in an unemotional and often times insensible way.