In the case of extremists such as Nazis, their hate speech can be suppressed without compromising anyone's moral integrity because the focus is on preserving everyone else's right to free speech, which authoritarians like the Nazis would brutally suppress were they in power. You only have to look at the likes of Trump and Farage to see this embryonic oppression in action as they peddle their alt-right lies and hate.

There is not moral argument for allowing these people to go unchallenged.