A teenager who killed a man with a single punch before going to a friend's house to play computer games has been jailed for life.The 17-year-old boy punched Simon Mitchell, 43, with such force that a witness described him falling over "like the minute hand moving from 12 o'clock to three o'clock."He delivered the killer blow at the very moment Simon outstretched his arm to shake his hand.The boy was found guilty of murder following a ?? day long trial at Manchester Crown Court, Crown Square. He was today, ??, sentenced to ??The youngster had already pleaded guilty to two other unprovoked assaults in which he'd punched two other men just yards away from where he landed the fatal blow in Bolton town centre.In the first incident, on the night of Friday, 7 June 2013, just seven days before Simon died the boy punched a 31-year-old man with learning difficulties in the face as stood at a bus stop on Blackfriars Street.The victim was knocked unconscious but regained it and was relatively unscathed.The boy struck again four days later, this time punching a 41-year-old man without warning after they struck up a conversation, again on Blackfriars Street.He punched him once in the face, knocking him through the open door of a waiting bus. He even joked: "I think he wants an Adult Day Saver." Again, the victim was relatively unharmed this time.This was to change on Friday 14 June when his path crossed that of Simon's.Simon, who had recently been made redundant, came into some money that morning when his brother gave him £200 cash. He arranged to meet up with an old friend and went into Bolton town centre where he went to several pubs.At around 9pm Simon, who was intoxicated by this stage, happened to walk by his attacker and his friends. They would often gather at Victoria Square where they would chat and drink alcohol. That night, the boy had already threatened to punch two other intoxicated men who had walked by.As Simon walked by, the boy and a couple of friends got up and approached him, striking up a conversation. Simon quickly became maudlin, talking about not becoming a father and two friends who had committed suicide.This struck a chord with the boy as his step-father had also taken his own life. He told Simon to change the subject, but when he carried on talking, his friends could see he was getting increasingly agitated.The boy's friends, sensing his was going to launch an attack, repeatedly told him to move away from Simon, but he said he was going to "bomb him" a reference to a surprise punch that knocks someone out.Simon offered his hand in friendship several times to try and defuse the tension, but the boy told him he would "bomb him" if he didn't go away.Simon asked, "what's a bomb?" and with that the boy punched him in the face with such force that it fractured his jaw. He fell horizontal, hitting his head on the hard ground.Instantly unconscious, it would have been obvious that he was in serious trouble and some of the offender's friends went to Simon's aid and called an ambulance.The defendant however left the scene and went to a friend's house to play on computer games.Meanwhile police and paramedics battled to save Simon's life but he was pronounced dead at the scene, about an hour after the punch landed.Police arrested the boy two days later. He told officers he acted in self-defence.Detective Chief Inspector Pete Jackson, of the Major Incident Team, said: "Simon lost his life at the hands of a young man who appears to have been driven by aggression and pent-up violence."Like all cowards and bullies he targeted people who he knew would be unable to fight back. Twice in the run-up to this tragedy he punched men in the face for no reason whatsoever."He didn't know them and he would have known they didn't want any trouble. On the night Simon died, the offender's friends had already twice had to step in to prevent him from hitting someone else."It was Simon's incredible misfortune to come across him at this point. Despite repeated attempts by both Simon and the defendant's friends, he would not be stopped from doing what he intended to do, punching a man he didn't know hard in the face."The fact his pathetically macho attack led to a man lying bleeding on the pavement clearly didn't move him. Instead of going to his aid he simply left to drink some more and play computer games."It's a callous way to end an innocent man's life and a timely reminder, as if we needed one, of the mortal danger of carrying out a one punch attack."