A barrister for Sean Hackett said the Tyrone teenager is guilty of manslaughter - but not murder - in what he described as "a very clear case of diminished responsibility".The defendant, a former up-and-coming star with Tyrone GAA, admits shooting his father dead last year.However, the 19-year-old has pleaded not guilty to the murder charge and claims he was mentally ill at the time.The jury at Dungannon Crown Court has heard how father-of-four Aloysius Hackett, 60, was shot twice in the head outside his home near Augher on 4 January 2013.Sean Hackett, the former captain of Tyrone GAA's minor football team, told prison psychiatrists he was depressed at the time and believed his life would improve if he put one of his parents in heaven to watch over him.On Wednesday, with evidence in the trial completed, the court heard closing speeches by prosecution and defence.Prosecuting counsel Ciaran Murphy QC urged the jury to return a guilty verdict.He said a diminished responsibility defence did not apply in this case because expert medical evidence showed Sean Hackett did not have a mental illness and was capable of understanding his actions at the time of the killing.Mr Murphy drew attention to Sean Hackett's decision not to take the stand and give evidence."The one voice that you haven't heard in this courtroom is the voice of Sean Hackett," he said.Sean Hackett has decided not to sit in the witness box and tell you what he was thinking, how he did it, how he planned it and what he feels about it.Prosecution QC Ciaran MurphyMr Murphy said the case "cries out" for Sean Hackett's testimony "but silence is what he's giving you".The prosecution said the defendant planned the killing with great care - choosing the rifle and practising with it, waiting for a moment when no witnesses were present, removing evidence from the scene and fabricating a story about his father being attacked.Mr Murphy said: "He trained to the extent that he was able to shoot his father twice in the head while moving... He had gained proficiency in that weapon."The prosecution told the jury that the defendant "liked the feeling of power when he had that weapon in his hands"."Killing made him feel more powerful," said Mr Murphy, "it made him feel more in control."The jury was told Sean Hackett did not call an ambulance after shooting his father.Instead he picked up spent cartridges, hid the rifle in the boot of a car and left the scene.Mr Murphy added: "He got feelings of excitement from what he did, but he had little empathy for his victim."That is who Sean Hackett is. He had a good family and a good home life, but he was a dangerous man."He left Aloysius Hackett slaughtered in a pool of blood at the back of his house and then came up with a cock-and-bull story to explain what he did."Manslaughter is the right, just and proper verdict.Defence QC James GallagherIn his closing speech for the defence James Gallagher QC told the jury the grounds for diminished responsibility had been clearly established and he contended that the medical evidence showed Sean Hackett did have a serious mental health condition.He said Sean Hackett may even have displayed the early stages of schizophrenia saying the accused is guilty of manslaughter, but not murder."Sean Hackett is not evading responsibility for his father's killing," said Mr Gallagher."Manslaughter is not a soft option. The maximum sentence for manslaughter is life imprisonment."Mr Gallagher said the accused had a clear history of depressive symptoms and irrational behaviour - including an attempt to strangle his mother Eilish, three months prior to shooting his father.Mrs Hackett has been in court throughout the trial supporting her son."Doesn't it say it all," said Mr Gallagher, "that when arrested for killing his father there was no recrimination against him by his mother?"She had lived the horror of it before. She knew the explanation of it."He urged the jury of six women and six men to reject the murder charge and accept manslaughter on the grounds of diminished responsibility."This is one of the most extraordinary cases ever to have reached a criminal court," he said."Manslaughter is the right, just and proper verdict."The jury is expected to be sent out on Thursday to deliberate on its verdict.