I think both positions can be argued. On the one hand, people can be exposed to the most vile ideas without turning into monsters, while others can take something entirely innocuous and bring it to violent extremes or be inspired by it to do terrible things. In this argument, the problem is with the person, not the idea. And yeah, the idea itself is not dangerous; it's the actions taken in line with that idea.
Ideas are dangerous, since they incite and inform the actions. Actions have to be triggered by something, they do not happen in a vacuum. Without the ideas, there is no action.
Dangerous ideas work best when they are claiming to solve a problem, and absolve people of personal responsibility in the process. Your life isn't what you want it to be? It's not your fault, it's the fault of Jews, Immigrants, Liberals, Atheists, Clowns, Islam, Black people, etc. Get rid of them, and your life will be better. People tend to prefer simple, easy answers to problems which are very complex and difficult; which is why scapegoating and demagoguery work so very well. And if you can tap into existing prejudices and preconceptions, even better. Religion excels at this, but it's far from the only ideology that does.
People who are able to see the problem for what it is, who are not looking for easy answers, and are not pre-disposed to abdicating responsibility for their own lives and circumstances, are less prone to accepting such ideas unquestioningly.
As for turning into monsters, well, that depends on how you define monsters. The Schutzstaffel were definitely monsters, as were the Nazi leadership. But the rank and file Nazi party members? The general German populace? How monstrous were they? Even the ones who didn't approve of what was going on very rarely took any action to oppose it.
And that, I think, is the true monstrosity. "The only thing necessary for the triumph of evil is for good men to do nothing." -Edmund Burke
And that is what is being advocated by the free speech extremists, for good men to do nothing while evil men promote and recruit and inflame and incite.
On the other hand, taking the obvious example of Nazism, it's clear that you have a bunch of otherwise decent people doing insane, monstrous things because they were driven by a terrible idea. I will note, however, that this idea had to be repeated and amplified for years before it came to maturity in that way.
Nazi-ism/fascist wasn't really around all that long; but it touched on existing prejudices and anger, and provided a pat, simple solution to what was a complex problem. And again, it was relatively few people acting as monsters, and a whole lot of people doing nothing to effectively oppose them.
And we see that today with all of the "fine people" wanna-be fascists going out and committing violence, the bulk of the population sitting back and doing nothing, and those who do fight back against the neo-Nazis and other white nationalists being demonized as "just as bad" as the people who want to exterminate entire ethnicities.
So, still hard to decide whether banning certain ideas is a good... idea. But one way or another, it opens a rather dangerous door, as who determines what's a banned idea can change really quickly, and those who find themselves on the wrong side of the law may not be able to stop it.
Except, as has been pointed out, ideas are not being banned. What is being proposed is restricting the ability for people to promote certain ideas --
which have repeatedly been proven harmful -- in the public sphere. We already do that for some dangerous ideas, and other countries have done so for Nazi-ism and other forms of white supremacism without sliding into totalitarianism, so why would that be the start of a "slippery slope" here? As others have pointed out, the slope isn't really all that slippery when you look at the actual history, and what is being proposed.
I'm not really comfortable with the idea of government censorship, however, I am comfortable with holding social medial platforms accountable for their actions regarding harmful content. Facebook and Youtube and others have long indirectly promoted white nationalist content, while suppressing criticism of it (Facebook in particular), because sensationalism drives views, and views drive advertising dollars, and that is how they make their money. And not just white nationalism, but any extreme sensational content that is not outright illegal -- child abuse, animal abuse, and so on. They're quite happy to have it as long as it makes them money, and damn the social consequences.
That's why Facebook and Twitter drug their feet for so long regarding the Russian troll and bots. The trolls drove viewership up, which looked good to advertisers, and they're not going to turn their backs on that kind of money without a very compelling reason. As long as the neo-Nazis are allowed a platform, they will exploit it, and Facebook and Twitter and Youtube will continue to make money off it.
