HALIFAX -- The lawyer for a former medical student convicted of murder said Monday his client should be granted a new trial, partly because his defence team was allegedly "betrayed" by one of their own.

Lawyer Ian Smith told the Nova Scotia Court of Appeal that a private investigator hired by William Sandeson's defence team turned out to be a "traitor" when he told police about a key aspect of the case and asked to remain an anonymous source.

"Hiding his identity from the defence was for an improper purpose," Smith told the court, adding that police kept the information secret and "failed to get any legal advice."

He said that when Sandeson's lawyer learned about Bruce Webb's actions in the middle of the murder trial in 2017, he asked for a mistrial -- but the trial judge refused. Instead, Justice Josh Arnold adjourned the case to allow the defence enough time to investigate what happened and conduct further cross-examinations.

Sandeson was later convicted by a jury of first-degree murder in the August 2015 death of 22-year-old Taylor Samson, who like Sandeson was a student at Dalhousie University in Halifax. Court heard Sandeson fatally shot Samson during a drug deal at Sandeson's apartment. He was sentenced in July 2017 to life in prison with no chance of parole for 25 years. Samson's body has yet to be found.

Smith told the three-judge panel Monday that Webb's actions amounted to "state misconduct," and he suggested there was an "active coverup" of a breach of litigation privilege.

In his role as a private investigator, Webb was told to interview two of Sandeson's neighbours, both of whom initially told police they didn't see or hear anything the night of the crime. But when Webb talked to them, they admitted they had seen a badly injured man in Sandeson's apartment.

Court heard that Webb was worried he could face an obstruction of justice charge if he didn't disclose the new information to police. That's why he told an investigator it would be wise for police to conduct another interview with the two men, which they did.

Crown lawyer Jennifer MacLellan said the trial judge took reasonable steps to ensure the defence could deal with the issue in court. She said the declaration of a mistrial should be seen as a last resort.

"It's a very significant and serious remedy," she told the court, adding that police had acted properly when approached by Webb. "What were they supposed to do, turn him away?"

She argued that the incident involving Webb resulted in nothing more than a slight delay in disclosure.

During Monday's hearing, Smith also argued that police acted improperly when they searched Sandeson's apartment three times without a warrant. He said police entered the apartment on suspicion that Samson may have been taken hostage.

However, Smith said that line of reasoning didn't make any sense because Samson had been missing for 68 hours and Sandeson was already under surveillance.

"There was no evidence that waiting (for a warrant) was impractical," he said. "They only had a hunch that Samson was present."

As well, Smith argued the police acted improperly when they failed to tell Sandeson he was under suspicion of murder during a lengthy interrogation that was supposed to focus on allegations of kidnapping and drug trafficking.

Smith closed his submissions by arguing that the verdict on the first-degree murder charge was unreasonable because the evidence did not point to planning with intent to kill.

He said the evidence allowed for the inference that Sandeson may have intended to complete the drug deal or that he was planning to steal the large quantity of marijuana Samson had to sell.

This report by The Canadian Press was first published Jan. 20, 2020.