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As far as I can tell, it stems from it being easier to escalate an argument by not being nice, which means, trying to win at all cost, leads to not being nice, or a bar fight. Some people get so used to it, that they see any attempt to stop it as a personal attack on themselves... which, technically speaking, it is: a safe space is telling them "we want nice people, not chronic bullies like you"... so they react in the way they're used to: by trying to start a bar fight.

I don't think society has particularly "fallen", looking back at history, there are a lot of examples, some much more gruesome than an argument on the internet, of people not being nice to each other. If anything, we're going through a process of transferring IRL behaviors to more virtual mediums, both good and bad, but the bad ones seem to have come easier.

I think it's been last millenium that I already wrote, the Internet would end up becoming a reflection of reality, at which point we'd be able to see the real problems and try to fix them. Well, we're almost there, still like one third of the world population missing from being online... (expect things to get worse before they get better)... but we're already seeing a lot of what otherwise used to be hidden behind closed doors, or restricted to a small geographic area. Which itself has brought new challenges, but at least it's making it easier to run sociological analyses, and trying to find some solutions.

they could just move along.

And that's how you know those people are purposely being malicious. They want to ruin it for others because they think its funny to ruin things for other people and there's no real consequences for their behavior.