Stalin was certainly paranoid. I will focus on three periods in his life. I will go over his personality in his to power, the purges, and the last years of his life. I believe an analysis of him during the war is an entire topic on its own.

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First during his rise to power Stalin portrayed all the signs of a relative outsider coming into power. As Glassman describes him Stalin was "stealthy, crafty, and Iron willed." These were the traits Stalin prominently showed in his rise in power in the 1920's.

Next up are the purges. Getty describes the incredible heightened amount of paranoia. This was described by Ellman as being for "war preparation". So the dictator was evidently attempting to prepare the country for WW2, which the purging of the army didn't help. So we can see that as Stalin solidified his power, his paranoia grew and grew, could this be the sign of an aging man attempting to hold on to his gains.

One event that certainly affected the psyche of Stalin was the death of his second wife Nadya. She committed suicide after a public disagreement with Stalin in the year 1932. Montefiore states the following:

"She crippled me" he said. They had never seen him so soft, so vulnerable.

pg 21 Court of the Red Tsar

It is not very hard to see how such an event could cause Stalin to protect what he has, which would lead to paranoia, and of course the purges.

Finally the last years of Stalin's life. I find "The Nature Of Stalin's dictatorship" by E.A Ree's to be an excellent source on the matter. He describes how Stalin became more and more aged and thereby began to decline in mental capability.

In his later years, this
increasing alienation from the upper elite was compounded by a personal
degeneration of Stalin’s own mental capacities.

pg 79 The nature of Stalin's dictatorship.

This lead to him becoming more of a typical dictator, instead of the phony collective leadership he perfered.

In his last years, Stalin adopted far
more classical dictatorial attitudes. The formal meetings of the Politburo
ceased. He cut back drastically on his informal meetings.

pg. 104 of The nature of Stalin's dictatorship.

Overall we can see that Stalin likely had some kind of personality disorder. Extreme paranoia definitely, and a possible case of sociopathy. However known of my sources claim he was ever diagnosed or that it was known for him to have a case of some kind f personality disorder. This was likely more out of fear due to Stalin's treatment of doctors that said or did things he didn't approve of.

Sources:

Stalin's rise to power by Leo M. Glassman

Road to Terror by J. Arch Getty and Oleg V. Naumov

The court of the Red Tsar by Simon Sebag Montefiore

The nature of Stalin's dictatorship by E. A Rees. The Soviet famine of 1932-1933 revisited by Micheal Ellman