The (Portland) Oregonian said John Lyle Roberts testified Tuesday in Multnomah County Circuit Court his wife Virginia had taken part in the planning of the February 2008 killing.
"I wanted to make sure everything was fast and efficient," said Roberts, who testified he and his 51-year-old wife had purchased special bullets for the alleged mercy killing.
Prosecutors allege Roberts, 53, had simply wanted to get rid of his wife, saying the slain woman had never been diagnosed with Lou Gehrig's disease, also known as amyotrophic lateral sclerosis.
Roberts maintains he was with his wife when she was diagnosed with the motor neuron disease. His attorneys have attempted to convince the court their client's actions do not represent intentional murder, instead attempting to portray the killing as an act akin to assisted suicide.
Roberts' daughter, Sarah, 30, told the Oregonian she supports her father's stance the 2008 shooting was a mercy killing.
"I believe it was a sacrificial act," she said. "She was the best thing that happened to him. He thought he was doing the right thing."