A TEENAGE killer who was caught when police traced a 3.5-kilometre trail of blood from the murder scene was yesterday jailed for life. Yung Fu-wing, 24, was stabbed to death just moments after saving his 81-year-old grandmother from drug-crazed knifemen Chan Wing-yin and Liu Ka-po. The killers, both covered in blood, then fled on bicycles to a friend's house. But as they rode away they left an incriminating trail behind them, the High Court heard. Prosecutor Jim Dick told how Chan, 15, and Liu, 20, burst into the elderly woman's stone hut in Tin Shui Wai, Yuen Long, on the afternoon of December 17 last year. One of the men grabbed Ms Choy around the neck and threatened her with a knife. But before the robbers could ransack her home Yung rushed in and fought them off. As the grandmother fled the house to alert police, Chan launched a savage attack on Yung. The court heard the teenager was high on the drug Rohypnol. Ms Choy returned to discover her grandson lying in a pool of blood on the floor with multiple injuries. Transport worker Yung, who had a two-year-old son, was rushed to hospital where doctors discovered nearly 50 cuts and abrasions. He died the following morning. When police arrived at the scene, they found a trail of blood outside the house which led to a hut in Tai Hing, Tuen Mun, where Chan and Liu were arrested. Air-conditioning mechanic Chan admitted he had been to Ms Choy's home, but denied murder and attempted robbery. The jury convicted him of both charges following four hours of deliberations. Mr Justice Leong sentenced the teenager to life for murder and five years to run concurrently for the attempted robbery. Liu, who had pleaded guilty to manslaughter and attempted robbery before Chan's trial, will be sentenced at a later date.