A Campbell River, B.C., man who admitted to stabbing his seven-year-old son to death has pleaded not guilty to first-degree murder because of a mental disorder.

Brent Allan Warren wept throughout proceedings in B.C. Supreme Court in Victoria on Monday as a videotaped confession taken just hours after his son was stabbed 27 times was played.

In it, Warren says he didn't want to kill his son Jonathan, but had to in order to save the world from evil.

Warren was diagnosed with bipolar disorder 10 years ago but had never shown signs of violence.

Outside the courtroom, defense lawyer Robert Mulligan called it "an unspeakably sad case."

"This is achingly sad," he said. "There's a legal aspect to this but there's a human aspect to it as well."

Police were called to a home on Alder Street in the northern Vancouver Island town in July 2011, where officers found a seven-year-old boy suffering from "serious injuries." The boy was taken to hospital where he later died.