The fact that you can't legally say anything any time you want does not imply that there is no legally protected right to free expression. It isn't a slippery slope to a dictatorship.
"We should gas all the Jews, rise up with me" isn't just free expression. It is also an act with predictable consequences.
The most likely consequence would be the antagonist getting shouted down and shunned.
If you have to worry about a pogrom breaking out as a result of inflammatory speech then you likely have deeper issues to worry about.
"We should gas all the Jews, rise up with me" isn't just free expression. It is also an act with predictable consequences.