Kansas City Chiefs linebacker and former Long Island high-school star Jovan Belcher was allegedly battling football-related head injuries and booze, painkiller and domestic problems when he snapped and murdered his girlfriend before killing himself in front of two coaches Saturday.

A pal of Belcher’s told the Web site Deadspin.com that Kasandra Perkins, the mother of Belcher’s 3-month-old daughter, had threatened to leave him for good amid fighting between the pair.

The couple had only recently reconciled after Perkins left their rented house in Kansas City with the baby at one point to stay with friends. Perkins had returned, but friends said the relationship was still volatile.

BELCHER’S MOM WILL RAISE ORPHANED BABY ON LI FOLLOWING TRAGEDY

It didn’t help that he was drinking every day and taking painkillers while dealing with the effects of debilitating head injuries, the friend said.

Chiefs chairman Clark Hunt said today that Belcher was “a player who had not had a long concussion history.’’

Belcher, 25, and Perkins, 22, had argued for the last time when she returned home late from a concert Saturday morning. But the Belcher friend said the concert was only a “tipping point.”

“This was the result of a long-term conflict,” the pal said. “She made it clear that she was leaving and would contact a lawyer’’ to fight for custody and child support.

Cops today revealed that Belcher shot Perkins nine times before committing suicide with a different gun. His mother witnessed the slaying; she had been in town to help Perkins with the new baby, sources have said.

Belcher’s mother, Cheryl Shepherd, will now take custody of the couple’s infant daughter and plans to return with the child to the family’s West Babylon home, where her troubled son grew up, his relatives said.

The kin said the baby was in another room when Belcher snapped and unloaded on Perkins.

“[Shepherd’s] taking it as anyone else would’ve taken it,” said Belcher’s cousin, Eric Oakes, 20, who lives in the mom’s renovated house where Belcher grew up. “She just lost a son. We’re all coming together.”

Oakes, wearing a game-warn Chief’s jersey with Belcher’s number 59 on it, said his cousin was his role model.

“[He’s] always trying to steer me right. That’s the only person I wanted to be like. A role model, basically my father. He’s the person who made me play football,” said Oakes, who played running back for West Babylon HS.

In Kansas City, relatives trickled in an out of the home that had become a murder scene.

“I think she was home alone a lot,” said Kristen Van Meter, 31, a neighbor who went to community college with the victim. “He was kind of quiet. he would come and go.”

When he was there, she said, there were lots of parties.