A man who described in graphic detail to an undercover police officer how he killed a business associate has testified that it was a case of self-defence.

George Allen, a 55-year-old truck shop owner, is on trial for first-degree murder in the killing of 44-year-old Garry McGrath, who vanished in February 2004.

McGrath's truck was discovered at the West Edmonton Mall after 12 days, but his body wasn't found until October 2005. It was wrapped in plastic and buried in a hole drilled on Allen's property east of Edmonton at Antler Lake.

'I realized then Garry had died on me.'-George Allen

Allen was secretly videotaped describing how he planned the killing to a police officer who was posing as a member of a criminal organization.

However, Allen began testifying in his own defence Wednesday and told a different version of events. 

McGrath came at him with a chainsaw: Allen

Allen testified he told McGrath to stop using his farm for a wood cutting business, and McGrath then attacked him.

McGrath came at him with a chainsaw, pinning him against the wall, Allen told the court.

Blinded by oil coming off the saw, Allen said he threw his work coat into the saw blade, jamming it, then beat McGrath with an aerosol can and a piece of lumber.

When he cleaned the oil from his eyes, he says he saw McGrath on the floor.

"I realized then Garry had died on me," said Allen.

Rather than calling 911, Allen said he ditched McGrath's truck at the mall then went back to the farm, rolled the body in plastic and hid it in a snow bank.

Allen said he had panicked because he didn't think anyone would believe him.

His testimony continues Thursday.