These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Peter Guerin admits that he based the story on his personal experiences growing up. Problem is, Peter grew up in the 1970s. By the late 1990s, when this story takes place, many legal and systemic provisions had been put in place that would explicitly prevent the events of the story from happening.
The symptoms Peter describes of ADD do not fit actual clinical symptoms of the disorder. David is probably closer to ADHD, but the "violent temper tantrums" he is prone to having are not a symptom either. David is also never shown depicting any of the normal symptoms of ADD, such as...oh...inattentiveness.
Todd Ianuzzi is a straight up crook in Beavis And Butthead. However, he is not is a child rapist, bigot, or hitman. He is also not the kind of person to happily invite people to pizza or burgers on his own check out of friendliness. Amazingly, Todd is derailed to be simultaneously too evil AND too nice.
The rest of the Fashion Club, including braindead Tiffany and painfully shy Stacey, are portrayed as just as evil and manipulative as Sandi.
Daria is less "disaffected, cynical, but intelligent teen" and more "cruel, sadistic, violent antihero."
Ron the Death Eater: Sandi on the show is self-absored and occasionally petty in regards to one-upping Quinn, but as shallow as a puddle. In the story, she is curiously elevated to a murderous, bigoted psychopath who runs for school president entirely to stop David from winning and taking away her "special rights and privileges." This escalates to violence pretty much immediately. By the end of the story, Sandi has several murder attempts on her hands in the pursuit of her miniature Third Reich.
Once again, Beavis and Butthead have transformed from utterly ineffectual losers to vicious sex offenders.
Strangled by the Red String: Daria and David spend roughly three days together before Daria decides that she loves him and sleeps with him.