Punjab Police has launched an investigation into the brutal killing of a leader of a Hindu organisation by his official security guard, police officials said on Tuesday.The 45-year-old victim, Manish Sood, president of the Punjab unit of the Akhil Bharatiya Hindu Suraksha Samiti, was shot dead by his Punjab Police security guard at his house in Sirhind town, 60 km from here on Monday night.Officials said the police security guard Som Nath shot Sood from close range. He pumped 15 bullets into the victim.The accused, who was arrested on Tuesday, was one of the three Punjab Police guards assigned to the Hindu leader for security.Officials investigating the incident said Sood and Som Nath had consumed liquor on Monday night together. Police were trying to ascertain what led to the murder.