A man who beat his neighbour to death after finding child pornography on his computer should serve six years behind bars, says the Crown.

Patrick Belanger, 28, has pleaded guilty to manslaughter for the July 2009 beating death of Leonard Wells, 63, in west-end Montreal.

Prosecutor Thierry Nadon told a Quebec Court judge the sentence would send a clear message to society that vigilante killings are unacceptable and that a man's pedophilic tendencies don't give citizens "a licence to kill."

Belanger will be sentenced on April 21.

He was initially charged with first-degree murder following the attack on July 25, 2009. He had been helping Wells to move and had asked to use his computer while the two were taking a break.

He flew into a rage when he stumbled across images of children being sexually assaulted. Belanger called 911 and warned he would take matters into his own hands unless Wells was arrested immediately. He began punching and kicking the man when police failed to arrive after 20 minutes.

Belanger called 911 a second time as Wells lay in a pool of his own blood. The victim was transported to hospital in a coma and died a month later.

Belanger later told the court the child-porn images triggered memories of his own father's tale of childhood sexual abuse. The defendant also said he was drunk at the time of the beating and he had been an alcoholic for several years.

But the Crown said the "horrific" and "revolting" images did not give Belanger an excuse to kill a man.

"We cannot allow people to mete out their own justice," Nadon told the court.

"(Otherwise) we would be promoting anarchy. Incidents such as this one ... do not give him a licence to kill."

The prosecutor said the fact Belanger waited 20 minutes to administer the beating was evidence the killing was premeditated.

Defence lawyer Julien Archambault countered Belanger had no criminal record and has been in alcohol rehab for two years.

He requested a three-year prison term.

Belanger told judge Louise Bourdeau he's a changed man and was "not thinking rationally" at the time of the deadly attack.

"I'll have to pay for what I did," he added. "I have no hard feelings towards the Crown. Justice must be served."