Before trying to answer this question, we need to realize that this sort of question is fundamentally unanswerable. It is impossible to know exactly what Stalin’s personality was as we do not have the testimony of any professional psychologists or psychiatrists who were able to work closely with him and come to understand him. It is also impossible to know if communism has to involve brutality as we have had a very small sample of communist countries to examine. Thus, any answer to this question is really a matter of informed speculation.
Turning now to the question, I would reject this particular either/or scenario. I do not believe that either Stalin’s personality or communism was solely responsible for his brutality. Instead, I would argue that Stalin’s personality tended towards brutality and that the particular circumstances of the Soviet Union also pushed him towards brutality.
It seems clear that Stalin, at least by adulthood, had a certain capacity for brutality as part of his basic character. This may have come about in part because his father was an abusive alcoholic. We also know that, as a youth, Stalin was involved in a fair amount of violence involving gangs of people of about his own age. In his early revolutionary days, he participated in armed robberies and other violent acts. These facts certainly imply that Stalin’s personality was one that could be brutal.
However, it is also likely that the circumstances in which Stalin found himself required some brutality as well. The Communist Party had to impose communism on people, many of whom were in no way eager to be communists. Leaders sometimes engage in brutal violence when they are trying to impose an unwanted system on a population. The Communist Party also lacked any real form of democracy or any legitimate way for power to pass from one leader to another. This meant that people who were leaders, or who wanted to be in positions of power, had to constantly engage in informal jockeying for power and position. Anyone who had power was a potential target for those below. This meant that people who had power typically had to be very careful so that they would not be removed from office by people who wanted power for themselves. These are factors that are not unique to communism. Any system that is imposed on people and that lacks formal methods for transferring power from one set of leaders to another is going to involve brutality.
In these ways, it seems that both Stalin’s personality and the communist system he was involved with were instrumental in causing him to engage in brutality.