I remember when Noam Chomsky wrote a forward (afterward?) to a book by an author who claimed the holocaust did not happen. Chomsky's belief was that if this book was censored because of people's outrage than our society would be no better off than Nazi Germany or Stalinist Russia. Hitler and Stalin, he says, were in favor of free speech as long as they agreed with it.
I remember the late, great Christopher Hitchens doing something similar by defending David Irving's right to publish holocaust denial books.