'Mob behaviour' led to fatal teen bashing A trio of teenagers who fatally bashed a 16-year-old boy in Perth got caught up in "mob behaviour" when a sham drug deal turned violent, a court has heard. Quinn De Campe was lured to a Balga bush reserve with the promise of a cannabis deal on December 22 last year but the teenagers he met planned to rob him. The meeting turned violent, with Quinn punched, kneed and kicked so hard it left a shoe imprint on his head and caused his attacker to limp. Quinn was found bleeding, bruised and unconscious, and within hours of being rushed to hospital, he was declared brain dead. He died the next day. His attackers were initially charged with grievous bodily harm but that was later upgraded to murder. Three of the teens last month pleaded guilty to unlawful killing - a plea bargain accepted by the prosecution. Along with another boy, who did not take part in the attack, they also admitted to stealing from the victim. On Wednesday during sentencing submissions, defence lawyers for two of the teens said they had not considered the possibility the sham deal could turn violent if Quinn resisted attempts to steal his belongings. One of the lawyers said his client felt sick once the "impulsive" attack began but he did not want to leave his friends for fear of appearing weak. The other said her client had succumbed to "peer group pressure." Another defence lawyer cited a psychiatrist who had described the dynamic as "a group cohesion that confused their sense of individual responsibility," saying there was "an element of mob behaviour." While her client had conceded he planned to hit Quinn if he didn't hand over $175 intended for the cannabis deal and other possessions, he wasn't expecting violence of the scale that occurred. The fourth defence lawyer, representing the boy who organised the meeting and stole from the victim, urged Judge Denis Reynolds to hand down a conditional release order on top of the 11 months behind bars he had already served. But Judge Reynolds said that wouldn't properly reflect the seriousness of the offence. After it was suggested the offending was in the mid-range of seriousness, he said it was instead in the worst category because Quinn had been lured to an isolated location and left there severely injured after being outnumbered by youths who showed callous disregard for his welfare. The prosecutor said it was senseless and persistent act of violence by offenders who believed they could act with impunity. Quinn's parents were suffering profound grief and despair, she said. "They've not only lost their only child ... but they will have a clear reminder every Christmas ... of the worst period of their lives," she said. Sentencing is scheduled for Thursday.