Dungannon Crown Court has been told by defence lawyers that the defendant was suffering from a mental disorder when he shot his dad in January 2013.Sean Hackett has claimed he wanted to place his father or mother in heaven to watch over him.But a consultant psychiatrist - acting for the prosecution - interviewed the defendant in prison.There Sean Hackett spoke about his heaven claims: "I nearly made myself want to believe that. I needed to do that ... I felt these thoughts felt like a good cover-up, but I didn't really believe it. It was an excuse."Prosecution lawyers have suggested the heaven story shows a killer thinking coolly and logically, creating a ruse to manipulate and mislead people into thinking he was suffering from mental problems and diminished responsibility.The court has heard Sean Hackett, 19, admits killing his father Aloysius in January 2013 at the rear of the family home on Aghindarragh Road, near Augher.He fired three high velocity bullets from a Czech-made .22 bolt-action rifle. His 60-year-old father was hit twice in the head and died at the scene.Whilst the defendant admits the killing he denies the murder charge.Defence lawyers have suggested his depressed mental state meets the legal test for diminished responsibility.On Thursday a consultant forensic psychologist who also interviewed Sean Hackett in prison told the jury the teenager had been profoundly troubled by pressures relating to his college studies, his GAA commitments, his grandfather's death and the break-up of his relationship with his girlfriend.Dr Philip Pollock said the accused had formed an idea that killing one of his own parents would provide a solution and a distraction from his unhappy life, and would also place the dead parent in heaven as a guardian to watch over him.The court was told that on 4 January last year Sean Hackett armed himself with the rifle and bullets. Twice he lay in wait to shoot his mother Eilish but couldn't go through with it.But later that night he hid behind a car in the yard, waiting for his father to come home from a meeting at the GAA club in Augher.Dr Pollock said: "He told me, 'I felt powerful. It was going to be the end of it ... I stood up and shot him ... I could hear him screaming no.' "The court heard how he approached his dead father, took his hand and "started saying sorry".In the prison interviews, Dr Pollock asked why he had killed his father. Sean Hackett told him: "At the time you wanted a bigger distraction from what was getting you down ... It acted as an escape from a life you weren't really enjoying ... I needed to do it."Dr Pollock said he believes Sean Hackett displays the symptoms of a major depressive disorder: "There was an abnormality of mental functioning arising from a recognised mental condition that substantially impaired the defendant's ability to form a rational judgement."Sean Hackett was a rising star in Tyrone GAA. He captained Tyrone's footballers to the 2012 Ulster Minor Championship.His mother, Eilish Hackett, on Thursday said that three months prior to the killing her son had tried to strangle her in the garage at the family home.After this incident, Sean Hackett visited a GP with his mother. He was not given medication, but counselling was recommended.Three months later, he shot his father dead.Dr Pollock told the court that the defendant had not been referred to the most appropriate service. The trial continues.