The key to understanding why they would say those hateful things is that their hate probably wasn’t particularly deep to begin with. It came on too strongly because of the shock of seeing a really bad crime, but was forgotten about just as quickly and drastically. Modern politics is based on the fact that people experience such strong but ultimately shallow emotions over things that don’t impact their personal lives. That are some exceptions, of course.
The difference between shallow and deep emotions is that people actually act upon deep emotions. How many of those people who were ranting about killing that woman actually went out and tried to kill her? None of them, which shows that they weren’t really serious to begin with. So the only thing you should be shocked about is how much bullshit the average forum-goer spews out of his or her mouth, not how hateful people are.
Also, this is the internet, where people regularly take out their personal frustrations.