10 Horror Movie Killers EVERYONE Predicted
6. Andy Richards - Sorority Row

Sorority Row's killer turns out to be protagonist Cassidy's (Briana Evigan) boyfriend Andy (Julian Morris), who attempts to kill everyone that knows about the prank-gone-wrong which led to the death of a sorority sister, Megan (Audrina Patridge), to ensure his future with Cassidy is safe,
That's... pretty daft as serial killer motives go given that nobody was pointing the finger at Cassidy anyway, and while it might seem random enough to make Andy an unpredictable culprit, that's not quite true.
There's a very telling line early in the film, when Andy asks Cassidy why she drifted apart from her sorority sisters after Megan's disappearance.
To some it might seem like a lazy way to get some expository dialogue across, yet there's a slyly knowing tone to Andy's question, seeming to suggest that he already knows the answer and is therefore likely the killer.