How did this killer asylum seeker hoodwink the authorities?

In 2018, a 16-year-old boy called Lawangeen Abdulrahimzai shot dead two men in Serbia with a burst of eighteen bullets from a Kalashnikov automatic rifle. Four years later he murdered again – inflicting a fatal stab wound on 21-year-old Thomas Roberts. Roberts, whose ambition was to join the Royal Marines, was killed because he had tried to break up an argument between Abdulrahimzai and another man on a street in Bournemouth.