Part of what makes the Great Purge so incomprehensible was its pure irrationality — both to its victims and contemporary observers, and to modern scholars. The purges were generally arbitrary, and almost entirely a function of Stalin's paranoid, capricious personality and political infighting at

Both Yagoda and Yezhov are likely to have felt indispensable in their times. So too would many others who were purged, but that sense of being untouchable would likely have been much greater for the People's Commissar for Internal Affairs (the head of the NKVD), responsible for actually carrying out the purges.

Yagoda, as NKVD chief at the start of the purges, was naive in at least one respect: it doesn't appear that he appreciated quite how wide Stalin and his allies wanted to cast the net in search of 'Trotskyists' and 'Zinovievites', or that he displayed enough zeal in his search for enemies of the people. The telegram from Stalin to the Politburo on 25 September 1936, ordering Yagoda's dismissal as People's Commissar, says that much (emphasis mine):

We deem it absolutely necessary and urgent that Comrade Yezhov be nominated to the post of People's Commissar for Internal Affairs. Yagoda had definitely proved himself to be incapable of unmasking the Trotskyite-Zinovievite bloc. The OGPU [the predecessor agency to the NKVD] is four years behind in this matter. This is noted by all Party workers and by the majority of the representatives of the NKVD.

Robert Conquest's parsing of the telegram and the decision to replace Pagoda in The Great Terror: A Reassessment seems to hit the nail on the head:

No one, of course, thought for a moment that Yagoda's removal indicated mere "incapability." It was at once noted in the Party that "it was not a question of dissatisfaction with his insufficiently active work ... this indicated political distrust of him."

Christopher Andrew says much the same thing in his second history of the KGB, The Sword and the Shield, arguing that Yagoda "was far less obsessed by Trotsky than Stalin was. Stalin's chief grudge against him was probably a growing conviction that [Yagoda] had been deliberately negligent in his hunt for Trotskyist traitors."

But the question remains: could Yagoda have foreseen that Stalin wanted the kind of tempo and breadth in the purges that occurred after his arrest? I'd argue that it's difficult to believe he could have.

In any case, the reins passed to Yezhov — who was appointed specifically because he was a loyal Stalinist and deemed more likely to carry out the purifying crusade Stalin believed the party needed. Yezhov personally engineered Yagoda's eventual arrest, trial and execution a year or so after his dismissal, and Yezhovschina (the most wide-ranging and damaging period of the purges) began.

Yezhov's own downfall in 1938 was equally predictable with hindsight — not least because he was the public face of the purges. Per Conquest again:

Not that everyone blamed Stalin. His skill in remaining in the background deceived even minds like Pasternak and Meyerhold. If men—albeit nonpolitical—of this caliber could feel so, it is clear that the idea must have been wide-spread. The fear and hatred of the population was concentrated on Yezhov, who was thus unconsciously making himself ready to be the scapegoat, the eponym of the " Yezhovshchina."

Conquest and others also make the argument that Yezhov, like Yagoda, was in his last days seen as having failed to maintain the revolutionary momentum of the purges — that he did not organise more public trials, or a second set of military show-trials. But there's also an argument made by some that he wielded power too widely and that Stalin came to believe that Yezhovschina had damaged the Soviet state. Those perceived failures opened the door to Yezhov's political rivals — not least Beria, who had been appointed deputy chief of the NKVD and who held no small grudge against Yezhov — to begin manoeuvring against him.

Beria at least had some of the benefit of hindsight: he had watched the purges unfold, and had watched both of his immediate predecessors fall from power along much the same trajectory. I would, however, point you to /u/Smilin_Dave's answer, because he's totally right that the case for Stalin having been poisoned is far from conclusive — not to mention the fact that Beria had powerful enemies in the Presidium, who were waiting in the wings after Stalin's death. In the event, even his alliance with post-Stalin premier Malenkov didn't prevent him from being executed in December 1953.

The tl;dr here is that it's a particularly difficult topic to speculate about, because Stalin was no kind of rational actor and the internal politics of the Soviet political/security establishments were incredibly complicated. My take is that both men used the NKVD to settle their own scores, as well as to pursue actual or perceived enemies of the state — and that both would have hoped they were untouchable right up until the moment Stalin began moving against them.

Firstly I think it should be noted that the evidence for Beria poisoning Stalin is fairly patchy. It is a mix of Beria's supposed claims (and usually him the context of him saying to the rest of the Poliburo 'you owe me' or 'fear me, I poison people') as relayed by Khrushchev (who was pretty busy dumping on Beria, and had a tendency to exaggerate). From memory there is a paper arguing that Stalin had warfarin in his blood at his time of death, but as well as being a poison, warfarin is also used as a blood thinning agent by doctors.

I can't really recall the details around Yagoda being removed but Yezhov's fall is briefly outlined in J. Arch Getty's The Road to Terror. I suppose the punchline of The Road to Terror is that Yagoda might not have seen it coming simply because the climate in the Communist Party was changing (in terms of what was acceptable opposition, what exactly constituted opposition and so forth) and Yagoda in effect didn't keep up, while Yezhov's actions/words were more in keeping with the general flow.

I think it fair to say that Yezhov didn't really know what was coming until he was given the terribly prestigous job of People's Commissar for Water Transport. This would serve to distract him from his NKVD post. A few months later Beria was made his deputy, making it pretty clear who was being set up as his replacement. Yezhov then sent a succession of letters to Stalin acknowledging that he might of made some mistakes but it was all because he was terribly busy doing what he thought Stalin wanted etc. This is contrast to other petitioners who insisted there must be some kind of mistake in their coming under suspicion.

So there isn't any indication he saw what was coming, or that he did anything about it, and his response once it was clear what would happen to him was more to reinforce his professions of loyalty to Stalin rather than trying to bargin or express bitterness.