Shane Russell said an enraged Joseph James Landry, 67, was determined to murder Phillip Boudreau on June 1, 2013, in Petit de Grat harbour in Cape Breton.
"I would submit to you that James Landry on June 1 was the straw that stirred the drink ... and the series of events that led to the murder of Phillip Boudreau," Russell said in his closing argument to the jury in the Nova Scotia Supreme Court.
"He was the first man on the field. He was leading the cause."
Part of Russell's argument focused on a key witness, Craig Landry, who told the jury Joseph James Landry was behind the decision to run over the 43-year-old Boudreau, whose body has not been found.
But defence lawyer Luke Craggs said Craig Landry's testimony was flawed by a series of strange, illogical statements and he urged the jury not to believe it.
A statement his client gave to police where Joseph James Landry says he fired four shots at Boudreau from the Twin Maggies fishing vessel were also discounted by Craggs, who argued the fisherman was taking the blame to protect others.
In the videotaped statement played in court, Landry says he believed he may have hit Boudreau on the hip.
In an initial interview with police recorded on June 7, 2013, Landry says he drove the Twin Maggies repeatedly over Boudreau's four-metre speedboat until it sank. But in a second statement heard Thursday, Landry says he made that story up to protect other crew members.
"I wanted to cover for my son-in-law and Craig," he said to a police officer after swearing on a Bible.
His son-in-law Dwayne Matthew Samson, the captain of the Twin Maggies, is also charged with second-degree murder. His wife Carla Samson, who is owner of the lobster boat and Joseph James Landry's daughter, faces a charge of accessory after the fact. Craig Landry, who is a third cousin of Joseph James Landry, is charged with accessory after the fact.
Dwayne Samson, Carla Samson and Craig Landry have yet to stand trial.
In his closing arguments, Russell highlighted Craig Landry's evidence that Boudreau insisted he wasn't cutting traps and he pleaded with Joseph James Landry to stop.
"James Landry persisted in the cause. He continued his pursuit in killing Phillip," said Russell.
Craig Landry was instructed by Joseph James Landry to tie the speedboat's bow line to the Twin Maggies so that it could be towed out of the Petit de Grat harbour, he said.
And after Boudreau managed to cut the bow line, Russell said it was Joseph James Landry who urged other crew members to continually run over the speedboat.
He argued the jury should accept Craig Landry's testimony that it was Joseph James Landry who gaffed Boudreau and dragged him out to sea, and then oversaw another crew member as the body was tied to an anchor and sunk.
Craggs said Craig Landry's testimony was unbelievable, adding it would have been difficult to use a gaff to hook Boudreau's body from the deck of the Twin Maggies.
Teams of expert divers were unable to find Boudreau's body at the location where Craig Landry thought it might have been dropped, said Craggs, calling it a wild goose chase.
"He (Craig Landry) wanted to minimize what he did and put other things on other people and add a few things to make his story sound more believable," said the defence lawyer.
He said his client's videotaped admissions were an effort to take the blame for the death and came after police told him younger crew members still had their lives ahead of them.
The jury will hear the judge's instructions on Friday.
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