LANSING — A Williamston-area man who shot and killed his mother as she slept had been hearing voices ordering him to kill her, his attorney said.

At the time of the fatal shooting, Andrew Willson, now 20, was in the midst of a major depressive episode, Willson's attorney, Stephen Milks, said Wednesday.

Lisa Marie Willson, 51, was found dead in her bedroom on Sept. 8, 2017 in Wheatfield Township. She had been shot once in the back of the head as she slept.

Andrew Willson called police just before 7 a.m. Sept. 8 and said he had come home to find his mother had been murdered, Ingham County Sheriff's Detective Charles Buckland said in a hearing that led to charges being filed.

He later told police he removed a .22-caliber Magnum rifle from a locked cabinet, then crept into his mother's room and shot her as she slept, Buckland said.

Hours before Willson shot his mother, they argued over a puppy he had brought home. His mother told him he would have to take it to his father's house.

Although the argument over the puppy was presented as a possible motive early on in the case, Milks said the puppy had no relation to the shooting.

Willson pleaded guilty, but mentally ill to second-degree murder and felony firearm possession Wednesday at the Ingham County Circuit Court. A first-degree murder charge was dropped in exchange for his plea.

No motive

The night of his mother's death, the voices repeatedly told him to kill her, Milks said. That, combined with a lack of sleep that night and a depressive episode, lowered his inhibitions to a level where he couldn't drown the voices out.

Willson's case is an anomaly among cases he has tried, Milks said Wednesday. The lack of a motive is boggling, he added.

Milks said the family told him Willson and his mother had a "wonderful relationship." They didn't notice any open hostility between the two, no more than a normal teenager's relationship with his mother, Milks said.

The warning signs weren't there, Milks said. His family knew he was depressed, but they never expected something like this.

"It's the difference between a movie showing a countdown on a hidden bomb and just hearing a sudden boom," Milks said. "There was no logic behind it.

"The entire wreckage of this situation is so far beyond anything I've seen in other cases."

Childhood cancer

Willson was diagnosed with stage 4 Hodgkin's lymphoma, the most common cancer diagnosed in teenagers 15 to 19 years old, in 2014. He was 13.

The cancer spread from his lymph nodes to his bones in his neck area, giving him the stage 4 cancer diagnosis. He went through chemotherapy and radiation treatment in late 2014, and has been in remission since 2015.

But doctors and psychologists aren't sure of the impact the cancer drugs — which essentially are "poison," Milks said — had on Willson.

Milks had three psychologists confirm Willson's mental health diagnoses, and even spoke to a chemical engineer to determine the effect the cancer drugs could have had on him.

The science "just isn't there" to explain the impact, if any, the drugs had on his mental state, Milks said.

'He still loves his mom'

Because Willson knew his actions were wrong, he didn't rise to the level of legal insanity, Milks said. It would have been reckless to take on that defense in a first-degree murder case, he said.

The case is weighing heavily on the family, Milks said. They already lost Lisa Willson, and now are faced with the fact that Andrew Willson killed her.

"Andrew is remorseful...he's 100% remorseful," Milks said. "He loved his mom. He still loves his mom."

Shortly after Willson was charged, the court ordered him to undergo testing at the state's Center for Forensic Psychiatry in Saline. He was found competent to stand trial in January.

There is no sentence agreement with the plea deal. Willson faces up to life in prison. He is set to be sentenced at 8:30 a.m. Nov. 7.

Contact Kara Berg at 517-377-1113 or kberg@lsj.com. Follow her on Twitter @karaberg95.

