I don't know that /r/askhistorians is necessarily the best place to get an answer to questions like this.
There's a trend, at least in the areas I study (Soviet, WW2, law) to characterize people like Hitler and Stalin as "psychotic" or "crazy." Now, I'm not a psychiatrist, or a psychologist, or a doctor. I've read parts of my DSM-IV and DSM-V when my professional obligations have called for it, but I am not an expert.
I think there's a tendency of humans, when confronted with something as horrible as the Holocaust, or the Holodomor, or the Great Purges, etc., etc., to characterize the actors who perpetrated them as "crazy."
I think as historians, it's important to point out that these men did not act alone; they were part of vast systems and had clear historical and political antecedents. They did not "irrationally"--as in, they acted in ways reasonably calculated to achieve their goals. Moreover, I think calling them "crazy" or "sociopaths" detracts from the true horrors that humans do. It is far more horrifying that normal people, people not different or less logical or less empathetic than you or I, are capable of such acts, than someone who cannot understand cause and effect, or cannot empathize with other humans.
Were their personalities somewhat "weird"? I suppose? There seems to be a great deal of evidence that Hitler was something of hypochrondiac. See "Adolf Hitler's Medical Care" by Doyle, J R Coll Physicians Edinb. 2005 Feb;35(1):75-82., for a medical paper examining that.
But I'll speak to Stalin, since I know more about him.
Was he a "sociopath"? I don't really believe so. Did Stalin kill huge numbers of people, including those close to him? Absolutely. Did he believe that lots of people were trying to kill him? Absolutely. Was he wrong a lot of the time? No.
Stalin was an ideologue. He believed himself to be a Marxist-Leninist, and he had very clear ideas about what that meant, and he wrote about it a great deal. The Marxist Internet Archive has a whole section on Stalin. Like many of the Old Bolsheviks, he was a prolific writer, "Stalinism" such as it was came from him. He clearly believed that in order to maintain power over the Soviet Union, he needed to purge political dissent from the Party.
Is it "sociopathic" to do it the way he did it? I don't know; I'm not a psychologist, and "sociopathic" is just a label we place on certain behavior, as I understand it. But I can't say that what Stalin did and the way he did it was irrational, or illogical, or not designed to achieve clear ends.
As historians and as people, I think it's important not to pathologize people who do things we don't like. I think it detracts from the clear historical and political antecedents of people and movements. Neither Hitler nor Stalin sprung from nothingness. Undoubtedly, they are two of the most studied men in history, certainly, dare I say, of the 20th century. We know where they came from, both physically and politically. Neither Hitler's anti-semitism nor Stalin's authoritarian implementation of Marxist-Leninism were new or without precedent. That these men did horrible things is undoubtedly true, but doing horrible things alone, I don't think earns you the label of "sociopath."
Brief Sources on Stalin:
Stalin: Court of the Red Tsar. Simon Sebag Montefiore.
Young Stalin. Simon Sebag Montefiore.
Stalin: Breaker of Nations. Robert Conquest.
The Three Volume Trotsky Biography by Isaac Deutscher.
Stalin's Wars. Geoffrey Roberts.
A World At War. Gerhard Wineberg.
All Stalin's Men. Roy Medvedev
Khrushchev Remembers. Sergei Nikitivich Khruschev
Khrushchev: The Man and His Era. William Taubman.
[edit to fix formatting for sources]