By Poppy McPherson, Kwang Jiraporn Kuhakan and Orathai Sriring THA UTHAI, Thailand (Reuters) – In the days before he killed 36 people, including 22 children stabbed as they slept, the former police sergeant behind Thailand’s worst massacre was firing guns in his back yard. For several nights the sound of 34-year-old Panya Khamrap’s 9 mm pistol cracked the silence in the sleepy village of Tha Uthai, neighbours said. It was the latest show of violence from the former police officer, once a village success story who became an angry, introverted man in a downward spiral, though still invested with…