I can understand that. Most people see a little of themselves in David. He is the anti-peggy sue. For 19 books kids dreamed of joining the animorphs. Then comes the David trilogy and there is a character that is an audience surrogate in a few ways. However, rather than being just another helpful member of the team, David reacts like anyone probably would in that situation. That is not to say that David wasn't a sadistic little creep. What I mean is that the audience sees that they are sort of David and that they would have done the same. This gets under their skin. They wish David hadn't made those mistakes because deep inside they know that they might have done just the same.