They were obsessed long before the movie. There was such a backlash from a big chunk of the fandom when RPattz was cast because the fans didn’t think he was right for the part.
Anyway, I haven’t seen any smart kids turning to black magic and devil worship because of HP. But I have seen smart girls swoon over Edward Cullen and not realize that watching someone through their window while they sleep is creepy and disabling vehicles to keep them from seeing other people is not cool and emotional manipulation is abuse. And it’s also not just Edward. There’s Team Jacob, who swoon just as pathetically over Edward’s equally emotionally manipulative “rival,” who also physically pushes himself on Bella. And Bella, who is supposed to be a “strong” female and a role model to the readers, has a hard time sometimes deciding which of her abusers she loves more.
So, again, it blows my mind that people can call it “just a book” when it’s sending a message like that.