George Orwell over time became fond of pointing out how polar-opposite ideologues are more similar than they realize. His essay Notes On Nationalism is devoted to it.
"The Catholic and the Communist are alike in assuming that an opponent cannot be both honest and intelligent."
Ironically, or fittingly, depending on your views, it turned out that Orwell was himself quite guilty of the same flaws he decried.
He railed against propaganda and misinformation, yet he himself lied about the fact that he murdered an elephant by writing an essay claiming he shot it during an Escaped Animal Rampage, his writing on the Spanish Civil War about Communist attacks on POUM ignored the greater political reality on the ground and POUM's recklessness and bad actions, as well as making the POUM militia seem more important than it really was. And after the war, he wrote out a list of suspected communists and liberals for the British Government of Information.
He also exaggerated his time in Eton as a Boarding School of Horrors by unfairly attacking his headmister while hiding the fact that he was a homophobic prefect who ratted out suspected homosexuals. His private diaries likewise reveal that he had considerable anti-Semitic opinions. In the end, Orwell too loved Big Brother.
As has been pointed out in histories of the Wars of the Roses Richard III was demonized by the Tudors as a usurper who illegally took the throne and killed of rivals with a better claim to the throne then him. Henry VII had an even worse claim to the throne, usurped Richard, then proceeded to eliminate others with better claims then him, which continued even in the time of his son Henry VIII.
They both had artistic ambitions (poetry and painting), both had criminal pasts and did time in prison (for bank robbery and treason), and both were vastly underestimated by the establishment until they suddenly broke through and revealed themselves to be much more dangerous than everyone believed (Stalin was a minor Bolshevik party functionary who managed to systematically take over, and Hitler was the leader of a tiny nationalist political party who the leaders of the Weimar Republic saw as a handful of disaffected nobodies). On a personal level, both were sincere ideologues despite surrounding themselves with utterly cynical and corrupt hangers-on. Both were anti-semites who initiated purges on Jews, Hitler did that throughout his life, Stalin in his final months before his death led his successors to immediately erase his final directives.
Likewise, despite leading highly brutal regimes, both of them commanded genuine loyalty (as well as fear) from their people who struck by them even when both leaders' policies backfired horribly on them, leading their home country to be invaded, Stalin with Operation Barbarossa which he made worse by his army purges and his misreading of intelligence, and Hitler who refused peaceful surrender despite countless opportunities to do so, well after the war was logically beyond any chance of winning. Both blunders were terrible and had horrible consequences for their citizens, and yet somehow both of them lucked their way through, Stalin moreso than Hitler who actually faced an attempted palace coup and assassination attempt.
The key differences is that Hitler was the creator and leader of his racist nationalist ideology and party, while Stalin was the successor (and a highly insecure one at that) of Lenin's. Hitler was a charismatic public speaker and socially a charmer, while Stalin was so bad at public speaking that he favored print and posters over recorded speech and was highly conscious of how he presented himself outside his small and carefully chosen circle. Hitler was a lazy bum who preferred the photo-op but hated the desk, Stalin was an obsessive Workaholic. Hitler or rather his party was democratically elected, while Stalin and the Bolshevik Party came to power in a revolutionary coup in a nation that never had any democratic institutions or systems.
It's also been pointing out that Hitler and Nazi Germanyas a whole shared a number of similarities with prominent features of American society and history, as well as The British Empire. Hitler drew a lot of his inspiration on eugenics and racism from Henry Ford, Oliver Wendell Holmes and Madison Grant, as well as Manifest Destiny. More than a few commentators have noted that Germany's Americophilia which preceded Hitler and even existed in Imperial Germany often saw Andrew Jackson's Indian Removal Program and the Westward Expansion as a model for their own plans of Drang Nach Osten, which involved conquering Eastern Europe, displacing the natives (Jews, Slavs, Romani people) and encouraging settlements in the East. There was an actual Commission set up by Otto von Bismarck, and their documents often referred to Jews and Poles as savages, a tradition according to a historian that "can be traced back to Frederick The Great, who likened the 'slovenly Polish trash' in newly reconquered West Prussia to Iroquois".
Sheila Fitzpatricknote The Russian Revolution, Page 140, 2nd Edition, Oxford University Press.: In functional terms, the relationship between collectivization and the Soviet industrialization drive had much in common with that between the enclosure movement and Britian's Industrial Revolution more than a century earlier...Marx, after all, had emphasized the suffering caused by enclosures and peasant uprooting in Britain...Soviet Communists may have felt some similar ambivalence about collectivization and the accompanying peasant out-migration, which was a bewildering mixture of voluntary departure to the newly created jobs in industry, flight from the kolkhoz and involuntary departure via deportation. But they also clearly felt defensive and embarrassed about the disasters associated with collectivization, and tried to hide the whole subject in a smokescreen of evasions, implausible assertions, and false optimism.
During the Greek Debt Crisis, Germany refused to give any consideration to Greece and its economy regarding its debts to the European Union, refusing to back away from austerity citing principles and probity. The French economist Thomas Piketty called them out by reminding them that Germany has never repaid its debts to other countries and indeed repeatedly recieved debt-relief measures from Allied nations to help them rebuild their economy, and furthermore has a history of extorting debt without ever honoring their committments.
This article on Cracked points out the surprising similarities between Gangstas and Rednecks.
This video points out how Christian & Islamic fundamentalists are quite similar in their views.
The Blue Code of Silence, in which police officers resist testifying against each other, is sometimes compared to the Mafia's Omerta code of silence.
A turning point in the Civil Rights movement happened when a breaking news bulletin about a racial march interrupted a documentary, showing police attacking the marchers. The problem was that the interrupted documentary had just shown Hitler's goons doing the same thing to the German Jews. Viewers found the similarities very disturbing.
Ancient Roman graffiti found preserved in Pompeii had the exact same subject matter as modern bathroom graffiti.
In his last radio speech before capitulation of the German forces in Norway, Vidkun Quisling lamented the Soviet victories in Europe and warned against a total Communist takeover, while pleading to Norwegian public to resist it. Less than three years later, in 1948, Norwegian Prime Minister Einar Gerhardsen held a famous speech, where he - warned against a total Communist takeover, while pleading to Norwegian public to resist it. Note of interest: Gerhardsen was elected prime minister at the time Quisling was executed.