She has brothers..
But seriously, I think it's likely that she's been abused at home, has been bullied at school and/or has severe anger issues (from both) and this was the simply the straw that broke the camel's back so to speak..
Everyone's got a breaking point, and when some reach theirs, they get sympathy, while others get demonized with calls for felony charges and probably worse.
As someone who had severe anger issues from being bullied, my breaking point was when my bullies hit me, which happened after I tried to ignore them and they refused to leave me alone thinking that I was scared of them, and the teachers and school administrators I told laughed it off and ignored it, regarding it has "child's play," "just a phase," or worse, "love" if my bully was male and said that I needed to ignore it and "toughen up."
This, I think, is the sort of thing that can either build up an overwhelming sense of helplessness or extreme rage in the victim. I used to feel helpless, until I realized that my bullies will continue hurting me whether I ignore them or not, and so I figured that if I'm going to be hurt, then it might as well have a reason and I started fighting back.
Their laughter stopped when the kid they said was "in love with me" lost a testicle because of an attempted sexual assault. Which, I'd later find out that he apparently had a long history of, since it was the reason he was expelled from so many schools. If it wasn't for the teacher who ran in after hearing me scream for him to let go of me and having seen that I kicked him in self defense, I would've been expelled too.
His mother didn't press charges because she knew about her son's history, and although she didn't get into why he was that way, she also didn't think that I should be punished since it was self defense. I didn't get into trouble with the school because despite the officials denying knowing anything about this, while trying to paint me as an "extremely angry and violent little girl who is the product of divorced parents and a danger to everyone," my mother and I was not only able to show proof that I did tell them each and every time this kid bothered me as well as the things he said to me, and their responses, but, she threatened to go to the BoE with the proof and sue the school if they didn't back off their plans to expell me. They did.
After this incident, I didn't have as many bullies, except for the guy in 7th grade, and he didn't bother showing his face around the neighborhood due to the fact that he got his butt kicked by a 6th grader (yours truly,) and the other bullies I had before left me alone or ended up becoming friends after realizing that I was shy, and not stuck up. My brother's bullies weren't much of a problem once they found out who he was related to.
That aside though, I honestly wonder about what it is that she was going through that forced her to build up her resolve to the point where a single act of stupidity caused it to break.
While some idiots like the boy in question will continue on grating someone's nerve until they get a reaction, the reaction here was very extreme in this case and I truly wonder what the true cause of this was. I don't think that people just "snap" for no reason.
I say this because at 17, she must've seen her fair share of idiots like the 15 year old, at school, but she didn't react to them as extremely as she did with him. He simply messed with her at the wrong time, when its likely that any other day, she would've told everyone that he sh*t his pants and got the entire class laughing at him.
I just think that there is some serious back story here, and before I get out my pitchforks, crucifixes, torches and holy water to exorcise this "demon" I'd like to know what the it is first.
Now, if it comes out that she just wanted to beat someone over the head with a stool, then I'll be front and center with pitchforks and torches in hand, but I just think that there is a lot missing from this story..