That's not fair equation given that Nazism tends to involve a lot more blood than Fascism does. If it was, the term Nazi wouldn't have to exist. Every society has restrictions on speech, even this one. Basically 99.99% of all humans who've ever lived are Fascists under your definition.
Before the last few decades, reasonable people could agree on what facts were and this was true when the Founders wrote the first amendment. The whole "everyone has a right to their opinion" concept would be bizzare to people prior to this generation. Like, what. Freedom of speech blurs that line and makes your last question very hard to answer in 2019. It shouldn't be hard to answer though, that difficulty is a consequence of not being able to say something is wrong because a large segment of the population thinks it isn't. Generally does speech cause harm? Is it harmful. Unlike offensiveness I'd hope there is little to no ambiguity(beyond mental health) to what constitutes harm.