Adolf Hitler was a historical villain mentioned a number of times in various Star Trek media.
Born in Austria in the late 19th century, Hitler grew up to become the leader of the German Nazi party, and absolute dictator of the Nazi state.
Under his rule, a Germany reeling from defeat in the first World War came very close to conquering the world, but was so corrupt and evil that they had to be destroyed at the costs of millions of lives. Hitler committed genocidal activities against the Jewish people, having millions of Jews murdered in concentration camps. As the allies closed in on his bunker in Berlin, Hitler committed suicide and entered the history books as one of the most evil humans who ever existed.
James T. Kirk would later compare Dr. Korby to Hitler, stating that Kobry's promises of immortality were of the same type that tyrants like Hitler used to make. Spock later stated that Hitler was another example in a long line of humans whose desire for absolute power corrupted them absolutely.
The misguided Federation historian John Gill would use Hitler's government as a template to help the Ekos. Feeling that a benignly run government would help the Ekosian people, he established a Nazi government on Ekos. While it worked at first, the Nazis of Ekos went down the same dark path that Hitler's original government did until James T. Kirk and Spock exposed the power mad Melakon as the corrupt influence that was twisting John Gill's vision. As he died, Gill admitted he was wrong to interfere in Ekosian society, and was wrong to create a Nazi state on Ekos.
Captain Jean-Luc Picard agreed with the faux-historian Berlinghoff Rasmussen that saving an endangered planet could allow a child on the planet to grow up to become the next Hitler or Khan Singh. Picard countered that such esoteric debates wouldn't help solve his immediate problems.
Due to temporal interference, Hitler was much more successful in alternate timelines.
When Na'kuhl temporal agents assassinated Russian leader Lenin in the early 20th century, the resulting chaos caused Russia to be a non-factor in the lead up to World War II. Free to concentrate on the western front, Hitler's forces conquered all of Europe and the eastern seaboard of the US. Hitler visited New York City in 1944, complete with promises to kill "financial profiteers." This timeline was erased thanks to the crew of the Enterprise (NX-01), who corrected the interference and enabled the timeline to reset.
Doctor Leonard McCoy later accidentally changed the timeline when he travelled back in time to the 1930s. He saved the life of pacifist Edith Keeler. Keeler subsequently convinced the United States to delay its entry in the war, which bought Germany the time to win the nuclear arms race. This enabled Hitler to win the war. This interference was corrected when Kirk and Spock followed McCoy back in time and prevented him from saving Keeler's life.