LONDON , England ( CNN ) -- The killer of `` Harry Potter '' actor Rob Knox has been given four life sentences and told he will be behind bars for at least 20 years . The parents of Robert Knox read a statement outside the Old Bailey after Karl Bishop was found guilty of his murder . A judge at the Old Bailey court in central London sentenced 22-year-old Karl Bishop on Thursday , the day after he was convicted of the attack on Rob Knox and four of his friends in southeast London last May . Bishop stabbed them 10 times in less than two minutes , the court heard . Knox , 18 , had rushed out of the bar after he heard that Bishop had threatened his younger brother Jamie but he ended up being stabbed five times , once in a main artery . He died in hospital later that night . The judge , Mr Justice Bean , told Bishop : `` You are at present a highly dangerous man , '' the Press Association reported . `` There is plainly a very significant risk to the public of serious harm caused by your committing further offences of violence . `` Because you had threatened his younger brother , Rob Knox was among those who tried to disarm you . He paid for his bravery with his life . `` The truth is that you simply could not care less whether you killed him or not . When you learned that you had killed Rob your only response was to say 'Yeah , sweet . ' '' Days before the attack , the actor had finished filming on `` Harry Potter And The Half-Blood Prince , '' due for international release in July , in which he played the role of Marcus Belby . He was set to reprise the part in future `` Harry Potter '' films . Knox 's father Colin told mourners at his funeral , including co-star Rupert Grint , that his son had been `` living the dream , '' PA said . The Knox family left the court without commenting , but earlier Rob 's mother Sally said of Bishop : `` Once he 's got his sentence and he 's gone , I will not waste my time thinking about him . `` I just think maybe somebody like him may have some kind of disturbed mind , which may not be due to the life he 's had , it just may be something in him . '' Knife crime in Britain is a political hot topic due to a spate of recent killings of mainly young people in major cities . UK Prime Minister Gordon Brown has urged tougher sentences for those caught carrying knives . He told the Daily Telegraph last month : `` By carrying a knife you are not only endangering the lives of others , but you are more likely to be killed , or end up in jail . `` We need to change the way young people think about knives , we need families and communities working together ... to get this message across and help stamp out knife crime and get weapons off our streets . ''
The killer of `` Harry Potter '' actor jailed for life , must serve at least 20 years . Karl Bishop , 22 , attacked Rob Knox with 2 knives in southeast London last May . Knife crime in Britain is political hot topic due to spate of recent killings .