Blame the 4chan community, and the widespread influence it has had on the Internet as a whole. Now, I'll admit, I spend a good amount of time on 4chan, since there are a lot of interesting topics and discussions in the "special interest boards", but the community there can be quite toxic because of the relative anonymity users there have.
Ironically, I'd say that probably a good portion of the userbase are on the spectrum themselves, since the site tends to attract a lot of "nerds" and people who are otherwise "outsiders" from society. A lot of these people probably don't even realize they are autistic, and they just throw around "autistic" as a synonym for "ret*d". I'm not saying it's right, but give people a space where people can vent their honest thoughts and feelings about things without even having to have an actual screenname, and you're going to see a fair amount of bile.
As far as sites where people do have to use actual screennames, well again, there's still a degree of anonymity, just a little bit less so, so people do tend to hold back more, but again, for many people this is enough for them to say what they honestly feel about things, without any regard for decency. YouTube is a notable exception, because despite the requirement for a screenname to post comments, and the recent changes that have encouraged people to use their real names to post comments, the comments section on many YouTube videos is an absolute warzone. Why? An absolute lack of moderation, combined with an extremely large volume of users.
On a particular note, Encyclopaedia Dramatica is a wiki written by as*holes, for as*holes. They claim that their goal is satire, but I'd argue that much of what's written on there is outright hate speech. There's a difference, and the writers there have no clue what that difference is.
TL;DR: It's the Internet. When you take millions, if not billions, of users and give them even the slightest illusion of anonymity, you're going to hear people say some pretty nasty things. The best thing to do is to ignore it, and if the site allows, report it and move on.