The deranged Long Island teen accused of fatally shooting his ex-girlfriend, Emily Finn, fumed she would not stay “loyal” to him — and raged in a rambling note that she was “a lying manipulative bitch,” court documents revealed.

Austin Lynch allegedly blasted Finn, 18, in the back of the head with a shotgun in his family’s Nesconset living room on Nov. 26, before turning the gun on himself in a botched murder-suicide, authorities said.

Finn had met with Lynch that day to return his belongings after their three-year relationship fell apart.

“She’s a psycho and the whole reason for this i f–king hate her and saying she wouldn’t be loyal when i left for the marines and she would consider herself single pretty much sums up who she is,” Lynch allegedly wrote in an unpunctuated screed.

“I have set my mind on leaving this place the day before my 18th birthday at the start of this whole episode i thought i could get through it i had hope[d] we would reconnect and i had people guiding my [sic] through this time i haven’t had anyone besides my recruiter text me in a week,” Lynch wrote.

“I thought i knew emily and who i was dating but she’s a lying manipulative bitch and yet she still acts innocent and how i’m the bad person,” he whined in the note.

The disturbing scrawl was found in the notes section of Lynch’s cracked-screen iPhone and extracted with a search warrant by Suffolk County’s Digital Forensics Unit.

It was allegedly written six days before Finn’s murder, according to court documents from the criminal case obtained by The Post.

Lynch also blathered that another boy had caught her eye, court documents showed.

The would-be Marine visited Finn in October during her freshman year at the State University of New York at Oneonta — but the trip ended with Lynch calling her a whore, and later dishing to an unidentified friend that he “wanted to show how angry he was,” court filings alleged.

Lynch then told his pal that he expected to be dead on Wednesday, Nov. 26 — the day he and Finn were set to meet.

A Charles Daly 12 Gauge semiautomatic shotgun allegedly used in the killing was also recovered from the scene.

Prosecutors claimed the gun was cleaned on Nov. 24 — two days before the crime — meaning Lynch had to choose to load two shot shells in the firearm before the ambush execution.

After shooting Finn, Lynch turned the shotgun on himself, but suffered only a minor injury to his nose.

Lynch appeared in court on Thursday, brandishing a long bandage along the profile of his nose but sporting no other apparent injuries after his failed alleged suicide bid.

Finn was buried Dec. 1 at St. John’s Lutheran Church in Sayville, where mourners wore her favorite color, pink, to raise donations for the National Coalition Against Domestic Violence.

Lynch was charged with murder and faces life in prison, if convicted.