A Richmond, B.C., man has been sentenced to 14 years in prison for setting a fire that killed his son and mother-in-law more than three years ago — the culmination of weeks of increasingly "threatening" and "intimidating" behaviour toward the family, according to the judge.

Neighbours phoned 911 after Surjit Singh Dosanjh, 40, lit the fire in the master bedroom of a fourplex in the middle of the night on April 13, 2015.

His mother-in-law and teenage son were sleeping on bunk beds in a room they shared. They both died of carbon monoxide poisoning, despite firefighters' efforts to revive them.

Dosanjh was arrested and charged with two counts of second-degree murder. He pleaded not guilty to both counts when his trial began in January, but was convicted of manslaughter in July.

'He was angry'

B.C. Supreme Court Justice Peter Voith said Dosanjh's "threatening and violent behaviour" in the days and hours before the fire were an aggravating factor in his sentence, which was handed down in October.

Voith's reasons for sentencing, which detail the case, were published online Monday.

One month before the fire, a woman — who can't be identified due to a publication ban — ended a long-term relationship with Dosanjh.

"He was angry ... His comments and behaviours when interacting with both [the woman] and others were erratic, irrational, and threatening," Voith wrote in his reasons for sentencing.

Surjit Singh Dosanjh was found guilty of two counts of manslaughter in connection to a fire at a Richmond, B.C., fourplex that killed his son and mother-in-law in 2015. (Felicity Don)

The behaviour grew "more volatile" the day or so before the fire. Voith said Dosanjh broke a window of his mother-in-law's vehicle and shouted at a police officer over the phone.

Eight hours before, the unnamed woman told police she was scared of Dosanjh and took her two-year-old son to a safe house.

Dosanjh was convicted on the lesser offence of manslaughter because Voith said he was not satisfied beyond a reasonable doubt that Dosanjh intended for his mother-in-law to die.

Voith also said Dosanjh didn't expect his son to be home that night. The boy had planned to spend the night at his grandfather's, but decided to go to the fourplex around 10:30 p.m.

'Few' mitigating factors

Crown argued Dosanjh should spend 18 years in jail for each count, less time served. The defence had asked for eight to 10 years, less a day-and-a-half for each day already served.

Dosanjh was sentenced to 14 years on each count, served concurrently. With credit for time already served, Dosanjh will spend just under nine years behind bars.

The justice said there were "few" mitigating factors with relevance, except that Dosanjh "was, in many senses, a good father [who] certainly loves or loved his children."