It seems dangerous to make proposing laws in a parliament illegal, don't you think? Anything should go in a parliament. I'm also not sure hate speech could be illegal when a member of parliament speaks in parliament, typically parliamentary debate is more widely protected.
Are you saying that it should be perfectly acceptable for legislators to propose a law e.g. to kill all men over 60 in gas chambers and that it should be treated seriously and let go all the way through legislative process?
Even more: that is currently the case in liberal democracies AFAIK. Any legislator can propose any law. It's then up to the legislative process to decide whether it should be accepted or not. (Possibly, a constitutional court could roll back laws that violate the constitution.)
My favorite solution: Those who vote for legislation found to be anti constitutional by the respective courts lose their passive voting rights and must leave Parliament immediately.