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, 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 of POUM ignored the greater political reality on the ground and POUM's recklessness and bad actions, and of course 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.
Joseph Stalin and Adolf Hitler were Arch Enemies, but they were also similar in a lot of ways, beyond the obvious of both being brutal dictators. They both had artistic ambitions (poetry and painting), both had criminal pasts and did time in prison (for bank robbery and treason), both were born on the fringes of their future empires (Georgia and Austria), 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). Politically, both were totalitarians who believed strongly in their respective ideologies - to the extrent that any means necessary to further their utopian cause, even the mass murder of millions, were justified. Most disagreements seem to have been a matter of degrees: While Hitler was an ultra-nationalist, Stalin was a moderate nationalist; while Staling took the extreme socialistic position in economic matters, Hitler was moderate. Both saw the state, and themselves, as parents and shepherds of the people, to shape their respective nations with great brutality if necessary.
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.