A jury at Dungannon Crown Court on Thursday accepted arguments by Hackett's lawyers that he was mentally ill at the time of the killing.They decided he wasn't guilty of murder on grounds of diminished responsibility.The 19-year-old is the former captain of Tyrone GAA's minor football team. He led Tyrone to an Ulster minor title in 2012. But after breaking up with his girlfriend he began to experience emotional and mental health difficulties.As his depression worsened he formed the idea of killing one of his parents, believing they would watch over him from heaven.In October 2012 he tried to strangle his mother, but she struggled free. Three months later, however, his killing plan would be carried out.On 4 January 2013, Sean Hackett's father Aloysius returned home from a night-time meeting at St Macartan's GAA Club in Augher.But Sean Hackett was lying in wait at the rear of the family home on Aghindarragh Road.As his 60-year-old father got out of his car, Hackett opened fire with a .22 hunting rifle. The victim was shot twice in the head and died at the scene.As his father lay dying in a pool of blood, Hackett did not call an ambulance. Instead he picked up spent cartridge shells, hid the rifle in the boot of a car and drove away.Initially he tried to suggest his father had been attacked by someone else, but within hours he confessed to relatives and was arrested.At his two-week trial in Dungannon, Hackett declined to testify. But the court was told he fully accepted that he planned and executed the killing.