The family of a 12-year-old autistic boy who was stabbed to death by his mentally ill neighbour says the child was let down by British Columbia's mental health system and his killing was "completely avoidable."

John Fulton disappeared from the steps of his Grand Forks, B.C., home last August. Two days later, Mounties discovered his body inside Kimberly Ruth Noyes' home.

Noyes' second-degree murder trial wrapped up last week when she was found not criminally responsible for the killing due to mental disorder. She was sent to a forensic psychiatry hospital where her condition will be reviewed after 45 days.

Fulton's family said little during the B.C. Supreme Court trial, but in an open letter this week blasted B.C.'s mental health system.

"This senseless crime was completely avoidable. Noyes' actions were completely deplorable but mental health's inactions are equally so," the letter reads.

Noyes was diagnosed in 2003 with bipolar disorder, a condition that causes manic delusional episodes as well as suicidal depressions.

Each time she was in a manic phase, her psychiatrist testified at trial, she was quite psychotic and completely rejected the notion that she was mentally ill. Noyes believed her dead father was God, that the devil was in her house and that God was coming in a helicopter to take everyone away.

Noyes' eldest daughter and other psychiatrists who had treated her testified about Noyes' delusion that she had to sacrifice her youngest daughter in order to resurrect her.

Fulton's family said in its letter that testimony at the trial made it quite clear that Noyes was a threat to the community and a danger to children, yet no one warned the families that lived at the low-income complex she called home.

"The local RCMP, mental health workers, psychiatrists, her doctor, and her family knew she was ill," the letter reads. "And despite her clearly disturbing behaviour the months before John's death, no one thought to hospitalize her. Where was the common sense?"

Fulton's family said it's not suggesting that people who struggle with mental illness should be locked away from society.

But when someone is uttering threats about sacrificing children, the family said, mental health should be charged with protecting the rights of the general public.

"The loss of this beautiful child has been devastating to our family," the letter states. "The way in which he was taken from us was unimaginable. The pain we feel will take years to lessen, but will never be fully gone."

Noyes had a troubled childhood.

Her mother died when she was very young and she took her passing badly. Noyes' father died the day before she was to graduate.

Three people in her extended family also committed suicide.

Noyes enrolled in the University of Calgary but dropped out. She eventually completed a business administration degree and worked as a bookkeeper.