Us living in society makes us forget a simple fact: a significant majority of people are bad people.
The people who write messages like those aren't doing so to defend or protect the "victim". They neither want to "punish" an evil-doer. It isn't like they actually care about injustice or the well-being of others.
They are simply enjoying making another person suffer. They see an easy target and mob together against them. It gives them a sense of power and superiority. It they can do so under a banner of "justice", even better. That way they can feel like "modern day heros" by simply sitting and writing in their computer and phones.
They also take the chance to slip in their jealousy and insecurities, like those classmates or the people from her middle school. That way they can convince themselves they hate her due to a justified reason, not because they themselves are horrid, spiteful people.
And, of course, they always do so from the anonymity. No risks, no consequences. Those kind of people would never have an one-on-one conversation with the people they rage against in the internet. At most, they would yell or throw things at them from the safety of a mob. Cowards, all of them.
The most sickening part is that, if the girl had actually killed herself, the very same people that caused it would immediately start claiming what a tragedy it was. They would jump right in the flood of "condolencies". And they would actually think themselves as good people for it. No that they would ever apologize or take responsibility for the hateful comments they wrote before.
That is, sadly, the world we live in nowdays.