Nobody wants to literally burn someone alive. But at the same time, nobody wants to live with a witch in their community. Alone, nobody would go so far; but there's comfort and safety in numbers, and the more people support a cause, the easier it is to jump behind the masses and throw sharp objects without being singled out. If you'd like to see how an entire village can murder someone for, say, being a homosexual, google "necklacing".
Also, your idea of abusive behavior completely negates the real experience of people being harassed online. Women bloggers often get threatening letters and tweets that are hidden from view, telling them they should be raped, murdered, and worse. Even just a majority of people telling you you are a bad person is enough to cause severe depression and anxiety. Without experiencing it yourself, you really have no idea how horrible it can get. And if Brendan actually had a history of mental illness, this could easily drive him to kill himself, as many people have committed suicide from online bullying over the years.
Also: Are you saying these Mozilla employees were wronged? I'm certainly not aware if they were, so I don't see how any of this is justified.