The Crown case is that Macdonald, who turned 32 yesterday, was quietly seething about Scott Guy's plans for the farm they worked on together with Mr Guy's father, Bryan, and about the amount of work Mr Guy contributed.
Scott Guy dropped a bombshell in May 2006 when he declared he wanted to inherit the farm, and expected that would happen.
"That shocked every member of the family who heard it," Mr Vanderkolk said.
While Mr Guy had been living overseas, Ewen Macdonald had been working hard on the property since he was a teenager dating Mr Guy's sister, Anna, who he eventually married and had children with.
Tension between the men flared over aspects of running the farm, how to rear calves and even the purchase of petrol for farm vehicles.
Fuelled by the fear of losing his livelihood and having his family's future jeopardised, Macdonald embarked on a course of vandalism with the intention of scaring Kylee Guy so much she would insist on leaving the farm, Mr Vanderkolk said.
A house on a trailer was set alight and Scott and Kylee's almost completed new home was extensively damaged, with windows smashed, plumbing fixtures ripped out and abuse written on the walls outside.
Notes were left in the letterbox that read, "Stay away from him Kylee you whore" and "You cheating whore what goes around comes around".
Macdonald had taken no responsibility for those, but admitted vandalising the property.
Another note followed the trashing of the house saying, "Now you know what it's like to lose something you love."
In police interviews, Macdonald said he was "working his arse off" and that the partnership was not equal, citing the time Mr Guy was spending with his family.
The court heard details of an "explosive" argument in September 2009 during which "neither man showed an appropriate level of self control".
Shortly before the shooting, the pair travelled to Southland for a farming conference where it emerged that Scott Guy was project oriented and Macdonald was focused on farming technology.
Soon after, Bryan Guy went to a separate conference and returned with plans that included some of Scott's ideas. Discussion on the plans was deferred until later in the year.
But early on July 8, 2010, under the cover of near total darkness someone - the Crown says it was Macdonald - opened fire on Scott Guy in his driveway.
The gates to Mr Guy's driveway had been closed - to trap him, the Crown says - so he got out of his ute to open them.
Mr Vanderkolk said Mr Guy was silhouetted by the ute headlight before the fatal shots were fired.
"The killer must have stepped out of the darkness, illuminated by the lights as he stepped into the beam."
Mr Guy fell to the ground with "a shotgun blast to his throat followed by a second shot to his face, hands and arms".
"He fell dead where he was shot, in the centre of the driveway."
As he lay dead on the ground, blood from his wounds flowed into a footprint next to his head - a footprint the Crown says was made by his killer who stood near his head and also at his feet.
The distinctive footprints are a key part of the prosecution case.
It is alleged they were made by a type of dive boot that Macdonald had been seen wearing but which was not in his possession when police searched his belongings.
The Crown alleged Macdonald went to great lengths to cover his tracks and fool police, including killing three puppies on the Guy property and asking police officers if he was the intended target of the shooter.
Asked how he pleaded to the charge of murder, Macdonald replied firmly, "Not guilty, your honour."
His lawyer, Greg King, told jurors there was no doubt that Mr Guy was murdered, that it was no accidental killing or manslaughter - "... this was murder".
"What the defence says this is the proverbial and classic whodunit, a murder mystery."
Mr King told the jury members it was not up to them to solve the mystery but to determine whether the Crown had done so.
He described the fire and vandalism as "foolish or nasty acts".
"But they do not, the defence says, make him a murderer."
More than 100 witnesses are expected to be called in the trial, which is likely to take up to six weeks.
