Sheffield Crown Court was told 73-year-old George Webb, known as Hugh, told police his wife had begged him to help her kill herself after years of suffering in pain from a range of ailments.
But prosecutors said although Mrs Webb suffered from a range of different medical conditions, she also had a "histrionic personality disorder" – meaning she would "always tend to over-dramatise any description of what she saw, felt, heard or experienced".
Neil Flewitt QC, prosecuting, said Mrs Webb had talked for years about killing herself and said her husband told police she had got the idea to do it with sleeping pills "as a result of viewing a Midsomer Murders TV programme".
Mr Flewitt said Webb had suggested to police his wife was the dominant character in their marriage, and had convinced him to help her to die against his personal wishes.
But the prosecutor told the jury they would hear evidence which was "not entirely consistent" with this description of the couple's relationship.
Mr Flewitt read entries from Mrs Webb's diary including one which said: "Hugh terrible bonkers – in flat – wants me dead...absolutely dreadful and I was feeling better – wants me dead."
He described how Webb killed his wife at their ground-floor flat in Wadsley, Sheffield, back in May.
The defendant said his wife was frightened of suffering another stroke, and regularly spoke of committing suicide. After watching Midsomer Murders, she decided to take an overdose of tablets prescribed for an anxiety disorder. Webb said he had reluctantly agreed to smother her if the overdose did not work.
He took her a tray to bed with two glasses of brandy and the tablets. She downed the concoction and went into a deep sleep, but when he feared she was coming round, he suffocated her.
After his wife died, he showered and had breakfast before telephoning his sister and then the emergency services.
A pathologist concluded Mrs Webb died from upper airway obstruction. He also found no other significant disease within the body.
Mr Flewitt said: "Beryl Webb died because her suicide attempt failed and the defendant stepped in and killer her. He recognises that fact and accepts he unlawfully killed his wife."
The court heard Mrs Webb had had both kneecaps removed because of arthritis in 1984, and was left walking on crutches or in a wheelchair for the rest of her life.
She had told other people she "did not want to carry on in her present condition".
Police found a book at the Webbs' home titled Final Exit: The Practicalities of Self-Deliverance and Assisted Suicide for the Dying.
Officers also found a letter written by Mrs Webb expressing a wish to be taken to a clinic in Switzerland to end her life.
The jury was told the couple had been married for many years but had no children.
Webb, of Luke Lane, Wisewood, Sheffield, denies murder.
The trial continues.