Three West Australian teenagers have each been sentenced to four-and-a-half years in detention for the manslaughter of a 28-year-old man, while the instigator of the crime was sentenced to five-and-a-half years.

Tauri Litchfield died in Mandurah, south of Perth, in March last year after four boys, aged 15 and 16, assaulted and chased him until he tripped over a wall and hit his head.

In sentencing on Thursday, Children’s Court president Denis Reynolds said the boy who tried to pickpocket and then punched Mr Litchfield – after he slapped the youth on the back of the head – wanted to show the older man that he was the boss.

“You likely felt demeaned (by the slap) in the presence of the group,” Judge Reynolds said.

Despite suggestions during the trial that Mr Litchfield had been agitated after celebrating St Patrick’s Day with friends, Judge Reynolds found he did not engage in any unprovoked violence.

“Different boys were being a real nuisance to him in one way or another,” Judge Reynolds said.

He said Mr Litchfield’s personal space was closed off by the boys, which would have made him feel intimidated.

Judge Reynolds said Mr Litchfield was “extremely fearful” for his own safety from the “pack” of youths and was trying to escape to avoid being further assaulted.

He said deterrence was an important consideration to prevent similar violence in the community.

“People should be able to walk in public alone,” he said.

The judge also noted that Mr Litchfield’s family and friends were enduring “everlasting pain”.

“Sadly, his death is real,” Judge Reynolds said.

Earlier, Mr Litchfield’s mother told the teenagers they could still change their lives.

“You have an opportunity to choose how to use the rest of your lives, to make a better future for your family, your community and your children,” she said.

Judge Reynolds, who questioned the level of remorse from each teenager, said he hoped the boys would take note of her comments in their rehabilitation.

Each boy has already been in custody for about a year – part of which was at the adult facility, Hakea Prison, because the Banksia Hill juvenile detention centre had been trashed in a riot – and will be eligible for supervised release orders after serving half of their sentences.

A fifth boy was acquitted of any crime against Mr Litchfield, while a charge against a sixth boy was dropped during the trial.