Hitler was an example of narcissistic personality disorder. Some of these people can be extremely destructive if you give them the slightest bit of power (they become "malignant narcissists"). Fortunately, most of them (like L. Ron Hubbard) never have the opportunity to become dictators. Other narcissists are not so bad but their need to defend the ego at all costs causes them to make a lot of bad decisions. Like MacArthur in WW II and in Korea, although he functioned brilliantly as proconsul to Japan where everyone deferred to him properly.
In the Soviet Union, Hitler could not have founded his own movement with its own ideology. He'd have been off to the gulag in the twinkling of an eye. He'd have had the pragmatic sense to join the party and possibly have found his way to the top, although I'm not sure he had the particular skill sets for that. If he did become party chairman, he'd have found a way to skew the country's goals and ideology in the direction that fit with his extreme narcissism. Cult of personality, sure, but with lots of public appearances and speeches (like Trotsky). He'd have found a way to target the Jews through "revising" Marxism-Leninism, but he'd have also had to target class enemies rather than racial ones. And unlike Stalin, he'd have developed a very aggressive foreign policy and prepared for an offensive war to spread the revolution (as he understood it). This would have involved a version of Trotsky's permanent revolution but without crediting Trotsky. Hitler as a Marxist ideologue would have been a lot more apocalyptic in his revolutionary vision that Trotsky ever was.
As Soviet leader, I don't think he could have done as much damage as he did in Germany. First, the Soviet Union was too primitive economically in the 1920s and 30s and Hitler may have lacked Stalin's vision of rapid industrialization to prepare for invasion. Plus Hitler would have been preparing for an offensive war of world conquest, necessitating a dominance in the air and on the sea that was beyond Soviet capacity in those years (they could have developed a better air force than they did under Stalin, but nothing that the Western powers couldn't easily catch up with and exceed). A Soviet Union gearing for offensive war would scare the Western powers far more than Nazi Germany did in OTL, and they'd start rearming earlier. Also, a non-Nazi Germany (no Nazism without Hitler and probably not even an overthrow of Weimar) would be among the nations preparing to fend off an invasion, and they'd probably be helping the Poles. An obviously aggressive Soviet Union in the 1930s would have no allies; every nation in Europe would be against them. In the U.S., the Republicans would support Roosevelt in preparing for war a lot earlier.