We'll never know the full truth. There were a lot of suspicions about the actual cause - was he murdered or poisoned etc . Also, statements following his death suggested a cover - up.
Oh gawd, well he was so damned paranoid that you can bet your life he wouldn't have allowed himself to die without ensuring there was some sort of 'conspiracy theory' behind it! I really think that in the dark reaches of the night he even weighed the pros and cons of a bitter conspiracy against him, by himself!!!
In his own filth, he had a stoke in his private apartment and all the staff and the guards were so afraid of him that they dare'nt knock and go in. As a result he lay for about 3 days dying a slow, agonising death.
Served him right!
Stalin from the official records died of a stroke but the man in charge of what became the KGB Beria was alleged to have stated that he killed him by poisoning to the foreign minister at the time Molotov.
Beria was certainly capable and there had been a falling out between Stalin and Beria before Stalin died and certainly Beria was concerned that Stalin was moving against him. But this could well have been made up as Beria was purged later and later that year executed by firing squad.
Still without a post mortem and very little chance of that no one can really say.