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David Callaway is the true main antagonist of Hide and Seek (though it's not revealed until late in the movies); for most of the film it is assumed that the murders and strange events of the film are the result of either his daughter Emily or her "imaginary friend" called Charlie.
Yet it is revealed that "Charlie" is actually a split-personality of David, caused after he caught his wife cheating on him. Charlie is treated as a different being by both David and Emily - embodying David's darker side "Charlie" is a Hyde-like figure who seeks death and misery.