A homeowner's assertion that his training as a military reservist prompted him to head outside into the frigid darkness and shoot dead a man trying to steal his truck makes little sense, his murder trial heard Tuesday.

Testifying in his own defence, Peter Khill agreed he could have stayed in his bedroom and called police -- but instead chose to go barefoot into the February night in a T-shirt and boxers, armed with a loaded Remington shotgun.

"It was instinctive," Khill, 28, told prosecutor Steve O'Brien.

"Regardless of your allegedly instinctive reaction, you're a civilian," O'Brien said. "Everybody knows about 911. You pick up the phone and call the cops."

"That's one option, yes."

Within seconds of stepping outside, Jon Styres, 29, a father of two, lay dying in the muck next to Khill's 15-year-old truck.

Khill, who lives in a semi-rural area a few kilometres from the edge of Hamilton, testified how his now-wife alerted him in the early hours of Feb. 4, 2016, to the possibility of intruders.

He said he got up and went to the bedroom window, where he noticed the radio light of his truck in the driveway was on. He said he wondered who was out there, if they were armed.

"It was a real-life threat assessment," Khill said. "I felt that I was being threatened and I felt that I was not in control of the situation."

Khill admits to gunning down Styres, of Ohsweken, Ont., on the Six Nations reserve about 30 minutes away, but has pleaded not guilty to second-degree murder.

He said he didn't call 911 because he was worried someone could get into the house within seconds.

Khill said his reservist training was still very much "relevant" to his life and certain triggers or situations could bring on aspects of what he had learned, an assertion O'Brien said he found hard to believe.

Calling 911 was the "obviously reasonable thing" to do, O'Brien said in repeatedly challenging Khill on why he had acted the way he did. The victim could have been a wayward teenager, not an armed insurgent, the prosecutor said.

"I thought my response was reasonable as well," Khill insisted.

At one point during his testimony, a distressed woman sitting with Styres's relatives left the court and could be heard wailing from the hallway.

Khill recounted coming around the back of the 2001 truck he had bought for $2,900, saying he could only make out a silhouette in the darkness leaning into the cab through the open passenger door.

"I basically said in a loud voice, 'Hey, hands up'," he testified, yelling loudly in the courtroom at his lawyer's request, and again later at O'Brien's request.

He said the person backed away from the vehicle, turned toward him, and appeared to point something at him which he took for a weapon.

"It was a life and death situation," Khill said. "I raised my shotgun, took it off safe. I fired at the centre of mass."

O'Brien said it was more likely Khill had startled Styres.

"The moment he started to move, you shot him," O'Brien said.

"Soon as I thought he had a gun, I shot him," Khill said.

After the shooting, Khill said he went inside to put down the gun, then went back out and tried to save the gasping man lying in the muck by the driveway, while his partner called police.

The case bears some similarities to the killing of an Indigenous man in Saskatchewan by a white farmer, who was recently acquitted.

In his opening statement, lawyer Jeffrey Manishen told the jury the case was about self-defence.

"It was not a matter of going outside to shoot; it was not a matter of going outside to protect a truck," he told Superior Court.

Manishen said jurors needed to focus on what Khill believed at the time, and would have to decide whether he acted reasonably.

Walter Sroka, a former reservist with 56 Field Regiment, testified military training is based on repetition.

"You do it so much that you don't have to think about what you're doing," Sroka said.

Sroka said soldiers are taught to aim at the "centre of mass" -- the upper torso -- and to fire at least twice to neutralize a target.

Jurors have heard one shotgun blast from Khill hit Styres square in the middle of the chest; the other hit him in the back of the shoulder before entering his chest.

The trial will hear legal arguments without the jury present on Wednesday.