Historians agree that Stalin's motives cannot precisely be determined. However, most also agree that it was not mental illness, but aspects of his personality, that motivated Stalin to kill millions of people in the Soviet Union.
A Mentally Ill Stalin?
While it has been argued that Stalin was mentally ill, it has also been argued that Stalin's deliberate, calculated actions do not point to mental illness as an accurate diagnoses. However, elements of Stalin's personality were contradicting. While he was a brutal killer, he was a thoughtful host. While he destroyed those who opposed him seemingly on whim, he spared other individuals who contributed to culture – for example, the writers Pasternak and Bulgakov. While these inconsistencies may suggest Stalin was mentally ill, any explanations for Stalin's motives require more complex examination of his personality.
Stalin's Desire to Divert Attention
Stalin's purges occurred during an economically disastrous period. One theory posits that Stalin was motivated by a desire to divert attention away from the scarcity of food and supplies by blaming individuals for the state's failings. Instead of the people finding fault with the government (at whose head resided Stalin), the people themselves could be blamed for their own suffering. Therefore, Stalin was exonerated from censure.
Stalin's Fears
Though Stalin ruled with fear, he may have also been expressing his own fears of being overthrown. Stalin was exceedingly careful about his personal safety, keeping bodyguards close and maintaining emergency exists in his private residences. Stalin could theoretically curb his fears of those who desired him removed from power by removing them first.
Power-Hungry Stalin
One of the most reasonable and comprehensive theories about Stalin's motives lies at the heart of Stalin's personality. A non-Russian from humble beginnings, Stalin was power-hungry and ambitious. He used the Revolution to climb to power, then calculatingly secured his position by purging leaders who had originally been loyal to Lenin. His desire to control who lived and who died may have been part and parcel of this lust for power, or it may have been the most effective way to frighten people into obedience and destroy enemies, or even potential ones.
Stalin's motives can be detected where the personal and the political collide. While Stalin may have not been mentally ill, he exhibited psychopathic traits, which lead to lack of inhibition in his actions.
References
Dziewanowski, M.K. A History of Soviet Russia. New Jersey: Prentice Hall, 1989.
Medvedev, Roy. Let History Judge: The Origins and Consequences of Stalinism. New York: Columbia University Press, 1989.