"Daddy's Gone Now..."

(Spoilers) After unpacking the boxes that are never unpacked, looking at his stuff, and notices that his journal is completely blank, it is revealed that Charlie, Emily's imaginary friend is actually David's twisted split personality. Flashbacks shows "Charlie" murdering Alison with a pillow for making out with another guest at a New Year's Eve party & pretending that she commit suicide in the bathtub, pushing Elizabeth out the window & receiving an arm scratch from her, and killing a butterfly in half after coming out of a cave.

Example of:
Not-So-Imaginary Friend