Well, according to Stephen Fry's 1996 novel Making History, you'd be wrong. History student Michael Young tries it with the help of Holocaust survivor Professor Leo Zuckerman, and finds out that Hitler wasn't the worst thing that could have happened to the 20th century. Not by a long shot.
Making History contains examples of:
- Conveniently Unverifiable Cover Story: Dr. Bauer's wife and son assume the identities of a mother and son murdered at Auschwitz, and use those identities to get from Germany to the United States, to a brother-in-law of the dead woman. He'd never met his real sister-in-law or nephew, so had no reason to doubt the woman and boy showing up claiming to be his relatives. The boy in question, formerly Axel Bauer, is now Leo Zuckerman.
- Godwin's Law of Time Travel: Turns out someone is going to lead the Nazis to war, genocide, etc., and that there are people who can play that role who will make the world a lot worse than Hitler managed.
- Hitler's Time Travel Exemption Act: Subverted in that yes, they do get rid of Adolf Hitler by preventing his conception. Played straight in that this does not prevent the Nazis, World War II, mass murders of Jews and other "undesirables", etc.
- Nice Job Breaking It, Hero!: The no-Hitler version of the 20th century turns out a lot worse than the version on historical record. This is mostly due to a much smarter, more rational charismatic sociopath taking leadership of the Nazi party.
- No Celebrities Were Harmed: Dr. Dietrich Bauer is clearly a fictionalized representation of Dr. Josef Mengele.
- Water Source Tampering: Michael and Leo try this, dumping a male contraceptive into the Hitler family's drinking water supply not very long before Adolf's conception. It works, in a Nice Job Breaking It, Hero! way. "Zuckerman" was born Axel Bauer, son of a German doctor who chose the SS, and in both versions of history wound up doing medical research at Auschwitz. In the alternate world, Dr. Dietrich Bauer figured out the contraceptive, which had also sterilized other men getting their water from the same source as the Hitlers, and used it to wipe out large populations. The spoilered info is in addition to the creation of a rather more efficient and effective Third Reich.