An unhinged Long Island teen charged with killing his ex-girlfriend in a botched murder-suicide was furious when she went off to college – and flew into a rage when she dumped him, Suffolk County prosecutors said Thursday.

“I have set my mind on leaving this place before my 18th birthday,” accused killer Austin Lynch wrote in a chilling manifesto after 18-year-old Emily Finn dumped him. “I f–king hate her.”

The disturbing new details came as Lynch, who survived a self-inflicted gunshot to the face during the Nov. 26 incident, wore green scrubs and bandages over much of his face as he was arraigned on a second-degree murder charge.

Assistant District Attorney Dena Rizopoulos said Lynch went from obsessive to downright scary after Finn left for SUNY Oneonta then dumped him two weeks b

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