It's one of the great questions of history, and indeed philosophy: what does it take to create a Hitler or a Stalin? What circumstances does it take to produce such evil?
Newly released diaries from one of Joseph Stalin's personal doctors suggest that in Stalin's case, illness could have helped to contribute to the paranoia and ruthlessness of his rule over the Soviet Union.
Alexander Myasnikov was one of the doctors called to Stalin's deathbed when the dictator fell ill in 1953, and in diaries that have been kept secret up to now, he claims Stalin suffered from a brain illness that could have impaired his decision-making.
"The major atherosclerosis in the brain, which we found at the autopsy, should raise the question of how much this illness - which had clearly been developing over a number of years - affected Stalin's health, his character and his actions," Myasnikov wrote in his diaries, excerpts of which were published in the Russian newspaper Moskovsky Komsomolets yesterday.
"Stalin may have lost his sense of good and bad, healthy and dangerous, permissible and impermissible, friend and enemy. Character traits can become exaggerated, so that a suspicious person becomes paranoid," the doctor wrote.
In another fascinating insight into the inner world of Stalin, the secret diaries of Lavrenty Beria, who had a reputation as one of the most unpleasant and bloodthirsty of Stalin's inner circle, were also published for the first time this week.
The diaries, excerpts of which appeared in Komsomolskaya Pravda, are likely to prove invaluable to historians as an insight into Beria's warped mind as well as into the inner workings of the Soviet hierarchy.
Beria's diaries refer to Stalin by his revolutionary nickname "Koba" and are filled with coarse language.
The entries start in 1938, when Stalin called on Beria to leave his native Georgia and travel to Moscow to work as the deputy to Nikolai Yezhov, head of the feared NKVD secret police and known as "the bloodthirsty dwarf". The NKVD had just conducted the "Great Purge", when hundreds of thousands of Soviet citizens had been shot.
Yezhov himself was shot in 1940 and Beria took over his position as head of the NKVD, becoming one of Stalin's most trusted lieutenants. The diaries show that he saw enemies everywhere and had no qualms about ruthlessly moving to have them arrested or shot.
He was also known as a sexual deviant, frequently trawling Moscow's streets in his limousine and picking out women who would be taken back to his mansion and raped.
The diaries occasionally show a softer side to Beria, expressing regret and wistfulness about the life he had ended up leading. "I like nature, and fishing, but when is there time for that now?" he wrote during the height of World War II, in 1943.
Beria's diaries also shed new light on events during World War II. When in August 1942 Winston Churchill travelled to Moscow to meet Stalin, the new allies were suspicious of each other, and Beria claims he advised Stalin that the best way to win concessions from the British Prime Minister would be to get him drunk.
After the visit, Beria wrote: "These are not funny times, but we have all had a laugh. Koba told me that my advice about Churchill came in handy. Churchill agreed, got completely drunk and lost the plot.
"Koba told us about it and laughed ... Afterwards, he said: 'It's good when you know the weaknesses of your enemy in advance'."
On the evening of May 10, 1945, the day after Soviet troops celebrated victory in the long and gruesome war, Beria notes that Stalin started crying.
"Again we spent the evening with Koba ... And again Koba was not like himself. He was even softer, and he even had to brush away a tear ... What a huge burden has been lifted!"
Stalin died in 1953, and Beria was arrested shortly afterwards and shot, before the Soviet Union began a gradual retreat from the bloody excesses of the Stalin period.
"I would suggest that the cruelty and suspicion of Stalin, his fear of enemies ... was created to a large extent by atherosclerosis of the cerebral arteries," Myasnikov wrote in his diaries. "The country was being run, in effect, by a sick man."