10 Great Horror Movies Ruined By Their Twists
8. All The Boys Love Mandy Lane (2013)
ATBLML should be higher on the list because the twist is so awful, but can't be because the rest of the movie isn't quite good enough to warrant it. Mandy Lane (played by Amber Heard) is the girl-next-door all the guys want. A group of her friends vacation to a remote ranch where the blood quickly starts flowing in this generic slasher; it's nothing new, but it's not so horribly upsetting either.
One by one the (admittedly annoying) teens are picked off in appropriately gruesome fashion. ATBLML reveals pretty early on that the titular characters old platonic BFFL is the murderer but waits until the climax to let the viewers know about the twist: sweet lovable Mandy is in on it. And wait for it - she then double-crosses him by reneging on their suicide pact. By this point, it probably would've been more satisfactory for them to just follow through with that plan.
Oh, and there is an old(er) farmhand character who was (kind of) looking out for the kids but failed to notice Mandy's homicidal actions. Once she dispatches her partner (with a log to the head), she and this random older gentleman drive off into the sunset for reasons, apparently. Pass.