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, has been 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 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.

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. 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 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 were called in to assist, the sheriff said.