As storytelling I do think it's gratuitous and a kind of tired technique: You take the dislikable, even downright nasty, character, show their nastiness for a good while so the readers thoroughly dislike them, and then you show the tragic tearjerking Backstory That Made Them That Way so the readers will feel sorry for them and change their opinion. It's overdone, it's heavy-handed, and it's neither as plausible nor as deep as people who use the device think. Incidentally, studies suggest that what most encourages selfishness and psychopathic traits is actually wealth and privilege.
The other thing is that I can feel sorry for someone without actually liking them. She has surely been treated horribly and unjustly. It should not have happened. And, she shouldn't be treated horribly or unjustly in the present either, and maybe there should even be some allowances made. But that doesn't mean I'm going to actually like someone who, in the present, is nasty and hostile.
Sad to say, if I'd been in her class I probably wouldn't have helped, or even really noticed what was happening to her. I was not sociable, and too busy dealing with being bullied myself to pay that much attention to other people's problems--although it hit me lighter than most because unlike her I had a supportive home life, and I also never gave much of a damn what other people thought, so the main problem was just trying to avoid getting caught and beaten up. I suppose indirectly my mere presence as another target would have helped a bit by diverting some attention from her. You can only pay attention to so many scapegoats.