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 done something to him,’ No, she didn’t.”

Now, the teen is spending his 18th birthday and Thanksgiving in a hospital bed, recovering from a gunshot wound to the face and awaiting murder charges.

He allegedly gunned down Finn inside his Nesconset home around 11:10 a.m. on Wednesday, Suffolk County Police said.

Finn was reportedly returning some of the boy’s belongings to him following their recent breakup — a heartbreak he was not equipped to handle.

“The frontal lobe is still undeveloped and not equipped to handle and process these emotions. I went through breakups when I was younger and they’re awful but I never contemplated doing anything like this,” the friend said, adding that he was “disgusted” by the boy’s actions.

“That’s ultimately what this was. The inability of a young kid to process this. It feels like the world is coming to an end.”

The teen purportedly had shown no signs of violence leading up to the murder.

“His parents are very, very, very angry. It’s not a situation of, ‘My kid, my kid., No they’re holding him accountable,” the friend said, adding that they haven’t been inside the house since the tragedy: “They can’t come home. How would you be able to walk back into that house?”

“Emily is dead. Her family is destroyed. These people lost their lives in an instant.”

Finn’s grieving family declined to speak when approached by The Post Thursday, but an online fundraiser launched in the wake of her death described her as “a true treasure of a girl.”

“To know Emily is to love her, as she became part of the fabric of the lives she touched in her generous and kind way. Through her many years as a dancer, the children she taught as she prepared for a future as a teacher, and simply as a friend to so many, she will be sorely missed by all who knew her,” the fundraiser read.

Police have not yet publicly identified the alleged killer, but said he will be charged with second-degree murder and arraigned at a later date.

The full extent of his injuries was not clear, but authorities listed him in critical but stable condition.