A bugged-out exterminator went berserk on the D train early Saturday, fatally plunging a knife into a fellow straphanger's neck after they argued over a seat.

Cops rushed to the train and nabbed the alleged killer thanks to a quick-thinking train conductor, who locked the doors and kept him inside the blood-spattered car.

"He stabs him right in the neck, possibly the jugular," a cop source told the Daily News. "Blood everywhere. Complete mayhem."

Bronx resident Jerry Sanchez, 37, got on the northbound train at Rockefeller Center about 2 a.m.

He confronted Dwight Johnson, demanding he move a bag from the seat where the attacker wanted to sit, a source said.

The 36-year-old Johnson, believed to be homeless, refused and punched Sanchez in the face. Sanchez pulled out a steak knife and stabbed him several times in the neck and face, cops said.

The more than two dozen terrified passengers rushed to the other side of the car, which arrived moments later at the 53rd St station.

One of the passengers notified the crew via intercom. The conductor locked down the car and contacted workers at the rail control center, who called cops.

A police source said Sanchez tossed the knife on the tracks before cops boarded the train. The weapon was later recovered.

Cops found the mad bug man sitting down.

"'What? I didn't do nothing,'" a police source quoted him as saying. "Like nothing happened, like he's just another passenger."

MTA officials praised the train crew. "They did a really good job not letting him out of the car," a spokesman said.

The victim died on the train, where pools of blood were left along the length of the car.

Co-workers at Terminate Control, the Manhattan exterminator service where Sanchez works, said the boss sent him home Friday morning for showing up late.

"But he took it in stride," one co-worker said. "Jerry's not someone to hold a grudge."

The suspect's brother, Louis Sanchez, told The News his sibling hurt himself on the job two weeks ago when he fell two floors after a floor buckled under him.

"He was in pain because he fell," the brother said. "He was talking about headaches and backaches, and he was taking some kind of medication."

Friends and neighbors said Sanchez had been acting strangely since his accident. They said he'd been abusing painkillers.

Serdar Ozel, 47, manager of My Place Family Pizza near Sanchez's home in Bedford Park, said the accused killer is a regular customer. "I don't know what's wrong with him, but he's been acting strange for the last two weeks," Ozel said.

Sanchez was charged with second-degree murder and criminal possession of a weapon. He was stone-faced and silent as he was led out of the 18th Precinct stationhouse in midtown.

Cops said Sanchez was arrested in 2004 for selling drugs.

Murders in the subway system are rare. Saturday's slaying was the second this year. Last year, two people were killed in the rail system.