For the first time, the prosecutor has publicly laid out the Crown theory for the possible motive for the murder of St. Albert couple Marie and Lyle McCann.  

It focuses on Travis Vader's alleged drug use. Vader is charged with two counts of first-degree murder.

"Our theory is that Mr. Vader, as a result of his involvement in the drug subculture, had accrued a number of criminal charges," prosecutor James Stewart said during legal arguments before Justice Denny Thomas Tuesday afternoon.

"He was desperate, he was starving, and it was in that condition he came across Lyle and Marie McCann and he killed them.

"Why would you kill two old people?" he asked. "We believe this is part of the answer."  

Stewart advanced the theory because he wanted to question the accused's ex-girlfriend Andrea Saddleback-Sexsmith about Vader's drug use. Defence lawyer Brian Beresh argued it wasn't relevant.  

'Great other evidence'

"There's this great theory," Beresh said, "but there's no other evidence."

Thomas decided it was relevant and allowed the prosecutor to ask Saddleback-Sexsmith about Vader and drugs.

Andrea Saddleback-Sexsmith

Andrea Saddleback-Sexsmith dated Travis Vader between January and April of 2010. (Facebook)

Saddleback-Sexsmith dated Vader between January and April of 2010.  

"When I started dating him he was in good shape," she testified. "He wasn't pale. He was actually healthy looking. He was normal. He was a good person to be around. He was competent. He had a positive outlook. He was well built.  He was strong, good looking."

They broke up in April because she thought Vader was seeing another woman.  

She said the next time she saw him was late at night on July 4, 2010, the day after Vader allegedly murdered the McCanns.

Saddleback-Sexsmith said she was shocked by Vader's appearance.  

"When I saw him on July 4, he was gaunt, he wasn't shaved. His hair was messy.  He was very agitated.

" And he was thin. Very thin."

'I was, like, hugging a bag of bones'

She estimated he'd lost up to 40 pounds since the beginning of the year, and estimated his physical changes began around March.  

"I went to go give him a hug. I was like hugging a bag of bones. Like squishy."

She said he told her he was starving.  

"I remember he said that for some reason he was on the run for four days, going through the bush or whatever.  He said he hadn't eaten in four days."

Stewart asked if she thought Vader was involved in the drug subculture during that time.  

"Yes," she replied. "Meth."

Saddleback-Sexsmith admitted she never saw Vader take drugs or saw him under the influence.  But she said she was convinced Vader had become a meth addict.  

On Tuesday Saddleback-Sexsmith testified she thought Vader was already wanted by the RCMP in the disappearance of the McCanns when she saw him on July 4.  

But the McCanns were last seen alive on July 3 and not reported missing until July 10.

Vader wasn't announced as a person of interest until later in July.

During cross-examination Wednesday, Vader's lawyer went on the attack, suggesting Saddleback-Sexsmith had a serious drinking problem and was likely drunk that night, so her memory was not reliable.

"I can function and I can remember," Saddleback-Sexsmith shot back. "I'm very smart. There's no reason for you to say the testimony I gave was untrue or deemed unreliable."

At one point, she broke down on the stand, wiping away tears.

"This is high stress for me," she said.