Edmonton's former police chief took the stand in a Red Deer courtroom and testified his troubled son has made him feel powerless.

John Lindsay was called as a defense witness Thursday at his son's second-degree murder trial.

Lindsay's son, Mark, admits he killed his former girlfriend, Dana Turner, in August 2011.

Dana Turner

Dana Turner's body was disposed of in a forested area west of Innisfail. (Supplied)

The issue before the court is Lindsay's mental state — his defense lawyer hopes to prove the young man was not criminally responsible for his actions due to a mental disorder.

Lindsay was adopted when he was two weeks old. His mother, Louise, testified she and her husband were told the baby's biological mother drank and smoked during her pregnancy.

Mark Lindsay was later diagnosed with attention deficit hyperactivity disorder and fetal alcohol spectrum disorder. 

He was the youngest of four children. The first sign of real trouble came around his 11th birthday, when he began snorting Ritalin supplied by a sibling.

He began to abuse alcohol and smoke marijuana and got kicked out of Grade 10 for poor grades and pot possession.  His relationship with his parents was not good.

"He always had the sense we were not on his side," Louise Lindsay testified. "That we were against him."

Just before his 18th birthday, Louise Lindsay said her son "just left and didn't come home." He wanted to live on his own. 

Mark Lindsay began to show signs of paranoia, the court heard.

"He told his dad and me that there was a plot to kill him and that the police were aware of this plot," his mother testified. 

The former police chief said he tried to do everything he could to convince his son there was nothing to fear. 

"I think I made the mistake of trying to rationalize what was irrational," John Lindsay told court. "I felt rather powerless, quite frankly." 

Their son began hearing voices in his head. By the time he was 21, he slept on his apartment floor in case his attackers got inside and he had to defend himself, or make a quick escape. 

Louise Lindsay fought back tears on the witness stand as she recounted a phone call from her son around that time.  He called to tell her he didn't need groceries anymore, or anything else. She said she found out later he had a suicide plan. Her son was hospitalized for a few weeks until his condition stabilized.

Over the next few years, Mark Lindsay was in and out of hospital. 

The young man began to cry in the prisoner's box when his mother recalled a conversation they had months before he killed Turner.

"He said the plot to kill me is coming soon," Louise testified.

She quoted her son's words: "I know I'm going to be killed and I'm prepared for it, and I know this is going to happen."

His mental breakdown led to a stay at Alberta Hospital. That's where he met Turner.

Despite his ties to law enforcement, John Lindsay said he had no idea his son had been convicted of assaulting Turner after the pair got out of Alberta Hospital.

The former chief said he found out about the conviction when his son called him from the Remand Centre near the start of his 50-day sentence.

Mark Lindsay murdered Turner just three days after he finished serving his sentence.

Hours after the killing, Lindsay visited his parents, who had no idea their son had just stabbed and strangled his girlfriend.

John Lindsay testified his son seemed tense and agitated that day. 

A few days later, Mark Lindsay asked his mother to meet for a talk.

"He was very upset," Louise Lindsay testified. "He felt very hopeless about his life. About how he was going to be able to function in the world."

She said she told him it made her sad to see him so distressed.

She said she surprised by his response.

"Gee, mom," he told her.

"I wish I could believe you actually thought that."