Freedom of Speech
Freedom of speech is at the forefront of freedoms that need defending, because while it is one of the most important, it is the one we are most comfortable making exception.
We are not born with all the knowledge of the past illustration, we have to each individually learn and make our own moral compass, this takes time, and friction, preventing this due to some limitation on what is allowed to be spoken (and thus implicitly thought) will only give these ideas more appeal.
Why is it so important, why is it so important we let fascists and misanthropists spout their hate, especially when they would silence the rest if they could.
There are three main reasons, first is that we should hold ourselves to higher standards as ultimately doing the same as they would leaves us as being the same as they are. Ostensibly this is something we do not wish.
Second, and perhaps most important to me, is this gives us a chance to contradict them, to engage with them (I’m not naive enough to imagine I can change their minds) but I can make others looking see them for what they are, and make the other spectators have an idea of how to contradict these ideas, both for themselves as well as for their engagement of others.
Third assuming silencing would stop where we want it, and it won’t be a double edged sword that will be used against us is also naive. It is either absolute or it is nothing.
This isn't to say we shouldn't tell people who are fascists that they are and that their ideas of humanity are atrocious, or that misanthropists were born 50 years too late and no one is sorry the world doesn't bend around their will. But it is important they be able to say what they think so that we can reply, and someone who is a spectator can listen.
Let everyone speak, and rebuke with reason those you disagree with.