I think part of the online abuse is down to the fact that the world has gotten so politically correct. If you say the wrong sentence in public these days you are now liable to lose you're job or end up in court.
While sexism and racism are both unacceptable, people are still going to tell jokes and make fun, without it being malicious. In the absence of this in the modern world, people have turned to anonymous comments online where there are no repercussions and they can be a lot darker with what they say. It is only human to have dark thoughts on a daily basis and behind a keyboard people are letting them out.