A young Halifax man has been sentenced to a decade in jail for a plot that envisioned using rifles and gas bombs to kill unsuspecting shoppers at a mall food court.

Randall Steven Shepherd, who was described in court as a disaffected youth who was a "cheerleader" of the 2015 plot, pleaded guilty to conspiracy to commit murder Tuesday in Nova Scotia Supreme Court.

"But for a Crime Stopper's tip (to police) ... Mr. Shepherd's co-conspirators may very well have accomplished their goals," Justice Patrick Duncan said as he sentenced the pale, thin Shepherd to 10 years, minus time already served, as jointly recommended by the Crown and defence.

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The judge said it's difficult to imagine a crime more damaging to a community's sense of peace and security, but also said he considered that the 22-year-old was not the main conspirator in the case.

A prosecutor said the killings could have put the city on the map as the site of a killing of innocent shoppers inspired by incidents such as the Columbine high school mass murders of 1999.

"This horrible plan would have changed the face of Halifax forever," said Crown lawyer Shauna MacDonald.

Lindsay Kantha Souvannarath of Geneva, Illinois, still faces charges in the plot to shoot shoppers at the Halifax Shopping Centre in the city's west end.

The Crown alleged the two accused, along with James Gamble, a 19-year-old man found dead in his Halifax-area home, had planned to attack the mall on Valentine's Day in 2015.

MacDonald said Shepherd had planned to kill himself before the attack and while he wasn't the main plotter, he was aware of the plan and participated in the conspiracy.

An agreed statement of facts says he planned to provide six bottles for Molotov cocktail bombs.

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It also says he went to the mall with Gamble and they filmed the site of the proposed attack, discussed how temporary walls in the food court might block some shots and "Shepherd indicated that if at least one person from his high school was killed the attack would be worth it."

In one of the videos, which the pair imagined would be played after the attack, Gamble calmly stared into the camera and says, "You're lucky I couldn't get any more bullets."

As the lawyers spoke Tuesday, Shepherd put his head down and stared at the desk. Given an opportunity to speak, he turned and looked at gallery before addressing the court.

"I'm deeply sorry this has happened," he said. "I wish James (Gamble) were here so he could speak for himself."

MacDonald cited terrorism cases in her submissions to the judge, and said the case has provided disturbing insight into the nature of disenfranchised youth in Halifax.

Defence lawyer Roger Burrill said it's a case of "extreme youth disaffection," and repeated that his client was not one of the main conspirators and was suicidal at the time.

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The agreed statement of facts includes statements from Shepherd saying he couldn't harm others, though he regretted he couldn't help Gamble — described as one of his best friends.

Burrill said the transcripts of conversations between the other parties in the case can be described "as nothing short of chilling," but his client wasn't aware of all the details of a plan with "psychotic dimensions."

"Nevertheless he was aware and he provided a degree of support with limited assistance within the parameters of believing his life would be over (before the attack)," said Burrill.

The court heard that police were tipped off through Crime Stoppers and went to Gamble's house, and spoke to him by phone before the 19-year-old killed himself with a single shot to his head with his father's gun.

The defence lawyer said Shepherd was "an outer planet and Mr. Gamble was at the centre of this. It doesn't mean he's not culpable."

Burrill said Shepherd had been through a period of social isolation and failed relationships with young women. He was dragged into Gamble's world and shared with him an interest in gore and horror films on the Internet, he said.

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The lawyer said a psychiatrist found Shepherd's personality had been affected by a dark, online world and developed an alternative persona on his computer.

Shepherd was arrested at the airport the day before the planned attack, where he had driven to meet Souvannarath, court heard.

The judge said there "is some hope for the future," if Shepherd receives treatment and resets the course of his life.

Souvannarath is expected to go on trial before judge and jury in May, MacDonald said outside court.