Answer by Erica Friedman, forged in the crucible of Usenet:
If you've ever attempted to join a new online platform, you've encountered hostile users. You say you prefer character Y to character X and the next thing you have 40 comments telling you to kill yourself. You walk outside your house and see a neighbor and wave at them, but when they go back inside, maybe they spend hours sending death threats to strangers. What the heck is with people? Why are they so hostile online?
Just as the right-wing have cultivated fear and intolerance in the TV-watching audience, hate groups have actively exploited the insecurities and biases of members of pop culture subcultures for the past several years, harnessing sexual frustration and social awkwardness into a deeply angry group of scared, lonely people poised to lash out.
But, to be honest, even before that, long ago, back in the early days of the Internet, it was obvious that there are just a LOT of assholes in the world.
In Why Can't Silicon Valley Fix Online Harassment? Jenna Wortham writes (and I co-sign, having been online since very early days of the Internet):
The web's earliest architects and pioneers fought for their vision of freedom on the internet at a time when it was still small forums for conversation and text-based gaming. They thought the web could be adequately governed by its users without their needing to empower anyone to police it. The web's founders underestimated -- or perhaps overestimated -- what would happen as the internet grew. It became an unwieldy place where guidelines were an afterthought, and in the process, gave rise to some of the worst behavior the offline world had to offer but with greater efficiency and scale.
Because the men who create the platforms we use are the *least* likely to be the brunt of, or affected in a real way by, online harassment, most platforms are built with an assumption that "we will all behave because this is our home and we all want to be here" - a premise that simply does not work. I really recommend the Wortham piece. It cogently describes everything I have seen online in the last 30 years.
In fact, on Usenet (where I was a moderator in a contentious martial arts forum), we often saw people whose sole desire was to just be nasty and rude to other people to denigrate their style or teacher or person. Even before that, on BBSs, it was common for people to come on to a BBS and just generally make themselves odious, because they didn't know how to behave, or because they could. (I wrote a piece about this back in 2011, in fact: Problem Users and the Problems They Cause, a piece I should update after this past year's fake news bots affecting people in ways they don't even understand, mobilizing their outrage to turn them into extremists.)
The principle "Information wants to be free" allowed piracy of IP to thrive and the principle of self-policing, libertarian platforms enabled horrible people to continue harassing behavior online without repercussion. The fact that most people don't "see" other people's bad behavior when it doesn't affect them does not help.
Some platforms, where there are now teams of people constantly fighting online harassment, first began life with a volunteer squad or users to do that. Imagine your town having only a group of volunteer police, without more than cursory training. Now they have a pro force and are still unable to stay ahead of the millions of users here who merely desire to insult, attack, harass, whinge, troll and/or generally be unpleasant.
When I was a child, we were trained to use a library. Today, it behooves us to train our children to use the Internet in a way that makes online hostility a relic of an ancient, barbaric past.