The upstate New York construction boss accused of gunning down a 20-year-old woman has been described by a neighbor as a “narcissist” with a “short fuse” who would constantly rage about people mistaking his driveway for a road.

Kevin Monahan, 65, was charged with second-degree murder after he allegedly fatally shot Kaylin Gillis when the car she was in accidentally drove up his Hebron driveway Saturday night while looking for a friend’s house.

“It’s shocking, but I’m not surprised,” Monahan’s next-door neighbor, Adam Matthews, told The Post on Tuesday.

“He had a short fuse. There was never any doubt he had a short fuse. I think he was a bit of a narcissist.

“He could do no wrong, but everybody else didn’t know jack s—t.” 

Monahan’s biggest pet peeve was trespassers, according to the neighbor.

“It was always, ‘People just drive up my driveway and they think it’s a road.’ He was very adamant people didn’t come up, and for a long time he kept the bottom of his driveway gated because so many people did mistake it for a roadway because it’s so wide at the bottom,” Matthews said.

Even police got confused about where to go when they first responded to the shooting, the neighbor added.

“If someone said that out of this whole road, something happened, who would be the first person who popped into my mind? Kevin. My knee-jerk reaction when I first learned of it, I thought somebody probably drove up Kevin’s driveway to settle a beef,” the neighbor said.

A second neighbor, who didn’t want to be named, described Monahan as an “irritable character, kind of ornery.”

“From what he told me, it seemed he presumed the worst in people,” the neighbor told The Post. “He was more cynical, just an irritable personality, someone who was easy to get into an argument with.” 

Gillis and three of her pals were looking for a friend’s house when they mistakenly made a wrong turn into Monahan’s driveway at about 10 p.m. on Saturday, Washington County Sheriff Jeffrey Murphy said.

They were trying to turn the car around when Monahan allegedly emerged from his home and fired two shots from his porch — one of which struck the 20-year-old woman.

The group fled the property and called 911 from the neighboring town of Salem, the sheriff said, adding the area where the shooting took place had limited cell service.

Gillis was pronounced dead when first responders reached the car.

The house the friends had been looking for was just half a mile further up the road from Monahan’s driveway.

The owner of that home, who also didn’t want to be identified, said his son had been having a small gathering that night and Gillis had been invited through a mutual friend.

“This is unbelievable,” he said. “And to have a f–king scumbag that lives down the road. Something like this never should have happened. This is absolutely ridiculous.”

The sheriff previously said Monahan had no reason to open fire on the vehicle — noting that no one had exited the car in the driveway before the shots rang out.

“There’s clearly no threat from anyone in the vehicle,” Murphy said at a Monday news conference, adding that it was “easy to get lost” in the rural area.

“There’s no reason for Mr. Monahan to feel threatened.”

When deputies arrived at Monahan’s home after the shooting, the sheriff said the suspect was allegedly uncooperative and refused to leave his home.

He was taken into custody several hours later after the New York State Police Special Operation Response Team was called in to assist, the sheriff said.

In an interview with CNN on Tuesday, the sheriff revealed Monahan had already obtained a lawyer by the time he emerged from his home to turn himself over.

“[He], quite frankly, has not shown any remorse in this case,” Murphy said, adding that the suspect has yet to make a statement about the shooting.

The suspect’s attorney, Kurt Mauser, rejected any notion of a standoff — insisting his client didn’t even know the gunfire had struck anyone.

“This was not a standoff,” the attorney said. “This was my client and I not knowing what was going on in terms of the charges that were pending.”

The attorney also told CNN that Monahan said three vehicles, including a motorcycle, had actually pulled into his driveway and they were “revving engines [and] coming up the driveway at a high rate of speed” prior to him opening fire.

Monahan claimed to his lawyer that it “created an atmosphere and a fear that there was menace going on.”

Mauser said he was informed of Gillis’ death and the charge his client was facing after he made contact with authorities. It was then he made arrangements for Monahan to surrender, the attorney said.

Investigators haven’t yet divulged what gun was used in the shooting or if they have determined a motive.

Monahan is currently being held at the Warren County jail.