A man who walked into a Toronto public library and fired a crossbow bolt into his father's back, then crushed his skull in with a hammer, has pleaded guilty to second-degree murder.

Zhou Fang, 26, was charged with first-degree murder of 52-year-old Si Cheng after the incident on the afternoon of Dec. 2, 2010.

Crown attorney Mike Callaghan says the prosecution accepted a plea to the lesser offence after considering that Fang was the subject of "long term and horrible abuse at the hands of his father."

Crown attorney Jessica Smith-Joy says Fang was the victim of deprivation, beatings and psychological torture from the time he was a baby.

A senior forensic psychiatrist who assessed Fang described the abuse as one of the worst and most chronic child abuse scenarios he had encountered.

The Crown and defence are asking the judge to impose a parole ineligibility period of 10 years when Fang is sentenced on Sept. 4.