I'm not sure why that is, exactly, but a credible hypothesis might be that by living in filter bubbles, people simultaneously become less exposed to the shared humanity of their opponents, and also rewarded by their in-group for the "righteous anger" they express towards those opponents.
After all, if you're sure that your opponent is going to lie or cheat or kill you, then what's so bad about lying or cheating or killing them first? It may even feel like you're de-escalating the situation if you "only" censor people who you think are going to kill (someone like) you. (Don't get me wrong, some calls to violence definitely shouldn't be spread, but I think that people are too quick to justify their censorship that way).
But as you said, currently all of our mundane day-to-day dopamine incentives in the West are aimed at the precise inversion of that. Not a good thing.
Consider: if one wishes to justify "nasty behavior", as you call it, using the principle of Free Speech and then tries to also demand that no criticism be returned -- then this individual is clearly non-serious about Free Speech concepts.
Yet I've had friends harassed by "free speech advocates" for offering professional criticism of bigoted ideas. It becomes very clear that the people who have managed to plant a flag on "free speech" as a thing that needs defending are simply using it for other means.
Those people are not serious about free speech; they are deeply hostile to it. If they were serious about free speech, they would welcome the criticism.
In my experience, it's not the criticism that free speech advocates complain about (plenty of weak phonies do, to be fair, because they are not secure in their principles or viewpoints). In my experience, free speech advocates complain mostly about the moves of private organizations against individuals' right to free speech. The bans from the social media "public square," (suddenly more important during Covid...) or when someone gets cut off of PayPal for dubious political reasons, these things do matter. They may not be strictly "unconstitutional," or even really illegal, but they are dangerous, and they are against the ideals of a free society.
And, I'd add: we need to respect the rights of the crazies to speak their mind. Yes, respect. Otherwise, you have people stuck in their crazy bubble, following only social media accounts that agree with them, and not listening to any criticisms of their points of view because everyone treats them like a horrible pariah and has never bothered to explain why their ideas are wrong, and they've never had a chance to truly argue their position (which is how you suss out what you actually believe).
And what do we get when we don't let people with bad ideas speak their minds freely, argue their position, and listen to arguments from others?
We get January 6th.