Each of these arguments presumes that if somehow, you'd managed to control the ideas of these people, then just maybe those terrible things wouldn't have happened.
The kinds of people that let ideas sway them into terrible actions aren't going to suddenly not find an excuse to do terrible things, they'll just look for another idea that justifies it.
You can't police ideas based on the worries of bad things happening because ideas exist.
People do shitty things and justify it with whatever excuses they want, trying to remove an idea is a pointless gesture and in the end impedes on the freedom of all because, something bad might happen if the wrong person hears the wrong idea.
And again comes down to:
'There's a moral authority that knows best and the rest of humanity is too irrational to be left to their own devices, therefore the moral authority must control the beliefs and ideas of the rest'
Are you surprised that arguments about which messages get amplified and which do not happen? That they happened historically and that they happen now? That Facebook (and other pieces of modern technology) have reinvigorated this discussion because they change the economics and effort and reach of people's actions?
One difference today is that we're having much more of this with just words in the private space and fewer legal or governmental ramifications... you could've gotten in a lot of official, thrown-in-jail-or-killed trouble for having the wrong ideas in the past!
Could you stop speaking in "what even is a dangerous idea" questions and put forth your own arguments? What do you picture your ideal world looking like, in terms of person A thinking one thing, person B thinking another, and person A's actions after person B starts trying to convince persons C, D, E... to take up certain positions... Let's use one of the "obvious" historical examples of a dangerous idea: "people like this are evil and we must kill them."