Christopher Garnier says he vomited and his vision blurred after watching off-duty police officer Catherine Campbell open her eyes and gasp her last breath during a sexual encounter in which she had asked to be slapped and choked.

Garnier, 30, testified in his own defence Monday at his murder trial, telling a Halifax jury he initially recalled little of what happened in the early hours of Sept. 11, 2015, and only began piecing things together after his arrest five days later.

He said he had nightmares and felt "different" in the days after Campbell died. But he did not remember dumping her body near Halifax's Macdonald Bridge, he testified, and thought maybe she was only missing when he saw Facebook posts about her disappearance.

Catherine Campbell, an off-duty police officer with the Truro Police Service, was found dead near Halifax's Macdonald Bridge in on September 2015. (CBC)

Garnier also faced cross-examination Monday, with a Crown prosecutor questioning elements of his story and asking why, when he was asked multiple times during his police interrogation whether Campbell had wanted to be choked, he had shaken his head no.

The Crown confirmed Garnier never mentioned Campbell's request to be dominated to police. The cross-examination will continue Tuesday.

Garnier is on trial in Nova Scotia Supreme Court for second-degree murder in the death of Campbell, a constable with the Truro Police Service, and interfering with human remains. He has pleaded not guilty to both counts.

'Unfortunate accident'

"I would do anything to change what happened, and if I could talk to her family I would say I was sorry," Garnier told the jury.

In his opening remarks Monday, defence lawyer Joel Pink called Campbell's death an "unfortunate accident" during "rough sex" she had initiated at a Halifax apartment. A psychiatrist will also testify about why Garnier can't remember certain details from that night, Pink said.

This court sketch shows Christopher Garnier lying on a table in a Halifax courtroom. Testifying at his second-degree murder trial on Monday, Dec. 11, Garnier left the witness box to imitate for the jury what happened on a pullout couch at the McCully Street apartment where police have said Catherine Campbell died. (J. Vincent Walsh/For CBC)

Garnier testified he and his common-law partner, Brittany Francis, had broken up and he decided to stay at a friend's apartment on McCully Street.

That night, he drank, smoked marijuana and later headed downtown, he testified. At the Halifax Alehouse, a downtown bar, he met Campbell and the two returned to the apartment.

It's there, Garnier testified, that Campbell asked if he was "into domination." He initially didn't reply, he said, because he was caught off guard and had never been into rough sex.

Campbell asked him to choke her and Garnier testified he put both his hands on her neck for about 30 seconds. He testified he would have stopped if she had asked him to, but she didn't.

They moved to a pullout couch in the den, where Garnier said he laid down on Campbell and put his arm across her neck. She then asked to be slapped, reassuring him it was all right, he said, and he did so three times with the back of his hand, without looking at her face.

"I was uncomfortable and embarrassed about what I was doing," he told court.

An evidence image from inside the McCully Street apartment where Campbell died. (Handout/Canadian Press)

Campbell then grabbed his arm, Garnier testified, and pushed it down on her throat. He said his arm felt wet and warm, and then he saw blood. He left to fetch a towel, he told the court, and when he returned, Campbell wasn't moving.

He said as he tried to pick Campbell up, shaking her shoulders, her eyes opened. Garnier said he could hear her gasp her last breath. "I could hear air come out of her lungs."

Garnier said as he stood up, his vision was blurry and he felt sick to his stomach. He never called 911 or tried CPR.

He said he was panicked and everything felt "loud." He remembers folding the couch mattress to get it through the back door and glimpses of the Macdonald Bridge, but nothing else until his friend woke him the next morning.

ID, mattress still missing

​That night, he made up with Francis, his common-law partner. But he said he didn't sleep much in the days after, and had an image in his mind of standing in a doorway looking down at Campbell. He also recalled an image of the bridge.

During cross-examination, Crown attorney Christine Driscoll said Garnier was thinking well enough to place Campbell in the densest brush near the Macdonald Bridge, where her body laid undetected for days.

"I wasn't trying to conceal," Garnier replied. "I don't know what I was doing."

He also testified he couldn't remember whether he knew Campbell was a police officer before they got to the apartment.

The Crown also said Garnier must have got rid of evidence at several locations because Campbell's ID, shoes and the pullout couch mattress are all still missing.

A police evidence photo of a green waste bin. (Andrew Vaughan/Canadian Press)

Garnier said he couldn't remember dragging Campbell into the backyard, but it's logical to conclude that's what he did, given he later saw himself in video surveillance footage from a business in the area.

He also said he lost his necklace, so he bought a new one so his girlfriend wouldn't find out he lost her gift to him.

Garnier said he searched the backyard of the apartment because he remembered it being there. The Crown said that contradicted his earlier testimony that his last memory at the apartment is folding the mattress to push it through the back door.

Garnier testified he could still only remember little pieces when he was interrogated by police following his arrest on Sept. 16, 2015, and was "trying to process what happened." He told police what he thought they wanted to hear, he said, and never mentioned what Campbell had said to him because he didn't think the officers would believe him.

Camping trip plans questioned

When asked by the Crown what he planned to do the day he was arrested, Garnier said he was going into the woods to be alone and to try to remember what happened.

Garnier testified he did not bring his tent because it had a broken pole. He said he forgot his sleeping bag and did not bring his camp stove because it was dirty. The Crown questioned his planning for the camp-out.

Christopher Garnier, right, heading to court Monday with his girlfriend, Brittany Francis. (Peter Dawson/Radio-Canada)

Garnier said he thought about moving Campbell's body, but not seriously. Garnier said he was carrying his passport because he recently lost his ID and could not afford to replace it.

The Crown questioned why Garnier couldn't afford to replace his ID, but had enough money to buy a replacement necklace. The Crown also suggested he took his girlfriend's SUV because it had more room to contain Campbell's body, which Garnier denied.

Research of antidepressants

Garnier also testified Monday he was on antidepressants for most of 2015 to help him cope with depression brought on after his grandmother died. He said he stopped taking them 10 days before he met Campbell.

Garnier told the Crown he wasn't violent but was seeing violent images; he said that's why was doing online research on the side effects of the antidepressant he was taking.

He said his father was on the same medication and had night terrors.

Pink told jurors in his opening remarks that a psychiatrist will testify why Garnier can't remember details from the night Campbell died and that he has "acute stress disorder," which explains some of his actions. The psychiatrist will also explain erotic asphyxiation, Pink said.

The trial is scheduled to run until Dec. 21.

Blair Rhodes was live blogging from court. Mobile users can view the blog here.