A homeless gamer fatally stabbed a teen during a fight over seats at a Queens Internet cafe — and then went right back to his screen as if nothing happened, cops said Thursday.

“The [killer] just sat at the computer and stayed there, playing a game on the Internet, until the police came,” said Paul Chen, a manager at the K&D Internet Cafe on Union Street in Flushing.

The dispute began at around 9:15 p.m. Wednesday — before the victim, Yangpu Fan, 19, had even arrived — when vagrant Paul Kim, 51, got into a fracas with a group of Fan’s friends, according to police.

Kim, a regular at the cafe, had been getting his fix when Fan’s pals had asked him to slide over so they could play the popular online multiplayer game “League of Legends,” police said.

Kim refused, telling the boys, “I’m spending my time.”

The teens repeatedly punched Kim in the head, and he allegedly pulled out what appeared to be a hammer and swung it while screaming, “You’re going to die!”

The group darted from the cafe but returned about a half-hour later with more people — including Fan, who picked another fight with Kim, cops said.

When Kim again refused to change seats, one of the teens told him, “F- -k you,” before more blows were thrown.

Kim was hurled over one of the computer terminals during the ensuing brawl, cops said.

Then he pulled out a blade and jammed it into Fan’s gut, according to police.

“The knife was very small, and nobody noticed he stabbed him,” said Chen, the manager. “There were 40 or 50 customers and nobody noticed.”

The friends took Fan to New-York-Presbyterian Hospital Queens, where he died.

Police arrested Kim as he calmly played on the computer.

Chen said Kim was a regular who would play overnight while chugging energy drinks.

“He’s been coming every day for the past few days,” Chen recalled. “He sits there all day playing games on the Internet.”

“He’s very polite. He always smiles when he buys something.”

Kim was charged with manslaughter and possession of a weapon.

“It’s really sad that someone had to die over a seat,” Chen said. “Those guys shouldn’t have ganged up on him, but he shouldn’t have stabbed one of them, either.”

Chen recalled that Kim would pound at least four energy drinks during gaming sessions.

“He’d basically been living here 24 hours a day, playing games,” Chen said. “He’d step out for some food sometimes, but he’d leave his stuff at his seat. It was always the same seat.”