The Long Island teen who killed his ex-girlfriend in a botched murder-suicide was heartbroken about their “puppy love” coming to an end, a family friend told The Post.

Emily Finn, 18, was in her first year of college, while the boy, whose name The Post is withholding, was making plans to join the military, leading the pair to end their teenage romance, the source explained.

“It was young puppy love that stopped making sense. It wasn’t going to work. Most of these young relationships don’t,” the friend, who declined to share their name, but noted they knew the alleged killer since he was 4 years old, said outside the boy’s family home Thursday.

“I don’t know where this went wrong and what led to such an extreme reaction,” he continued.

“People are in the comments like, ‘she must’ve do

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