A grandfather drowned his nine-year-old grandson in the bath and then calmly sat rolling a cigarette until his family returned home, a jury has been told.
Lincoln Crown Court heard how Stewart Greene, 65, killed Alex Robinson at the boy's home in the city two days before Christmas after his mother had gone out to buy a turkey.
Prosecutors told a jury how Greene had a history of admissions to mental health establishments and hated the idea of living independently at accommodation arranged for him.
Michael Evans QC, prosecuting, described how the defendant had assaulted medical staff before at times he was due for release.
Mr Evans told the court how Greene was left alone with Alex on the morning of December 23 last year. He said the youngster, who was on the autistic spectrum, had been playing on his Xbox when his mother, Joanne Greene, left to go to Tesco.
Mr Evans said Ms Greene returned to find her father sitting calmly on the settee.
According to the prosecutor, Greene told his daughter: "I've drowned Alex in the bath."
The prosecutor said Ms Greene shouted for her son and "the fear began to rise within her".
Mr Evans said Ms Greene rushed up to the bathroom where she saw her son lifeless in the bath water, surrounded by his sister's toys.
He said: "She dragged Alex out of the bath and remembers his lips were blue and he was lifeless."
The prosecutor said Greene sat calmly in the lounge as frantic efforts were made to save the boy.
He told the first police officer to arrive that he had drowned Alex and was arrested. He later said at the police station: "I can't do prison. Am I going to hospital?"
Alex's mother, Ms Greene, entered the witness box to give her evidence after the prosecution opening but became visibility upset when the defendant began shouting from the dock.
The defendant was taken from the court and the jury was asked to leave.
Greene, of Danes Court, Grimoldby, Lincs, denies one count of murder. The case continues.
Mr Evans said it was the prosecution case that Greene ran the bath and the murder was pre-planned. He said the bath was the last place Alex would have gone willingly because part of his autistic tendencies was that he was very wary of water.
The prosecutor said Alex, who was 4ft 6ins tall and weighed four stone 12lbs, had a strict washing routine and would never lie in the bath, insisting on standing as he bathed. He even hated water on his face.
Mr Evans said: "The defendant forcibly drowned Alex in the bath."
He said: "His intention was clear and he did so in the most brutal way."