A man who drowned his nine-year-old grandson in the bath has been jailed for a minimum of 22 years and told that his “callousness cannot be overstated”.

Stewart Greene, 65, of Grimoldby in Lincolnshire, admitted murdering Alex Robinson at the boy’s home two days before Christmas last year. When members of the family returned from grocery shopping they found Greene sitting calmly on the settee, and he told his daughter: “I’ve drowned Alex in the bath.”

The grandfather had claimed Alex’s death was as a result of diminished responsibility and denied murder. But he changed his plea eight days into his trial at Lincoln crown court.

Sentencing Greene on Thursday, the judge Mrs Justice Thirlwall said: “During the course of this trial you have sought to manipulate everyone in the courtroom. You manipulate people to get what you want. This was not the product of mental illness, it was wholly your responsibility.”

The killing happened 12 days after Greene had been discharged from a mental health unit in Lincoln with no care plan in place. Two months earlier he had been identified as a danger after brandishing a knife at a woman and trying to strangle a doctor, the court heard.

Alex’s mother, Joanne Greene, told the court that she had pleaded with staff at the psychiatric unit not to let him out, fearing that he might attack someone again.

Greene has a history of depression-type illness, hated the idea of living independently and wanted to remain an inpatient, the court was told.

Alex was playing on his Xbox in the living room of his home when his mother went out Christmas shopping on 23 December. Greene locked the doors, ran a bath, walked his grandson into the bathroom and drowned him. The court heard he then rolled a cigarette and waited for the family to come home.

Two psychiatrists gave evidence during the trial saying that they did not believe Greene matched the criteria to mount a defence on the basis of diminished responsibility.

One of the psychiatrists, Dr Philip Joseph, described Greene as a “callous” killer who had done everything to try to remain in hospital when he was not suffering from any mental illness.