A security guard stabbed his girlfriend to death after he found out she had shown an interest in a man they worked with who was almost 10 years his junior, the High Court heard yesterday. The day after killing his girlfriend, Poon Man-sum, 32, chopped their 20-year-old colleague Chan Tai-wai, whom he perceived to be a love rival, at work and chased him for 180 metres before being restrained by colleagues, prosecutor Nicholas Adams said. Police discovered the body of 24-year-old Law Yuk-yu in her public housing flat the day after her death. She was found under a blanket, a fruit knife stuck in her back. Poon, 32, pleaded not guilty to murdering Law, a colleague who became his lover, in her flat at the Tai Wo estate in Tai Po on November 28, 2012. But he admitted attempting to murder Chan by hacking him three times with a cleaver the following day. Adams said that by the time of the fatal attack, Poon and Law were 11 months into their relationship, which he described as an open secret among their colleagues in security at Coastal Skyline, a private housing estate in Tung Chung. "It would seem that Ms Law had taken an interest in Mr Chan," Adams said. Adams said that an autopsy showed that Poon stabbed Law four times in the back. The injuries suggested that Poon had withdrawn the knife and pushed it back in again. One of the stab wounds punctured her lung. Adams said that after Poon killed Law, he tried to cover up the crime by using her mobile phone to send messages to her sister and his friend. The prosecutor added that Law's friend Lee Chung-wai called her mobile phone three times at about 6pm on November 28, but the calls went unanswered. Adams said Poon sent a WhatsApp message from Law's mobile phone to Lee saying: "All is well. I want to be left alone." "It's not all is well. All is not well," Adams told the court. The next day, Poon went back to his home and took a chopper with him to work, Adams said. In a changing room, the court heard, Poon chopped Chan three times. Chan ran away but Poon gave chase before he was restrained by colleagues. "Don't stop me. I want to die," Poon said when he was subdued. "He stole my girlfriend. The girl was killed by me already. I must chop him to death too." The court heard that Poon had no injuries. Adams asked the jury to decide whether it was possible he killed Law in self-defence or was provoked into killing her. "The prosecution says it's very clear that he intended to kill her. There is no accident," Adams said. The trial continues before Mr Justice Alan Wright on Monday.