I’d always understood that he suffered a stroke in his bedroom and his aides were terrified to check on him because he was a late sleeper. By the time they found him it was too late and he died days later.
I had always understood the story about the stroke, but that some of Stalin’s deputies (including Krushchev) would not permit anyone to enter his room for a long time...and, just by coincidence, many of them were likely to have been put up against a wall and shot.
Alternate reports have him being poisoned, to prevent mass executions among the Soviet leadership, with the same intentional delay for doctors to have access to him.
Any way you cut it, this guy was EVIL.