Crane driver Dempsey Nibbs, 69, then said he beheaded the mother of his two children, Judith Nibbs, 60, because he thought she was a “snake”.
However, the Old Bailey murder trial heard Nibbs had shown no signs of mental illness in the wake of the horrifying killing at the couple’s home in Hoxton, east London, in April 2014.
Prosecutor Crispin Aylett, QC told jurors to “brace themselves” as he outlined the gruesome case.
He told how the couple’s relationship had soured in the spring of 2014 as Nibbs suspected his wife of having affairs.
The victim had confided in her sister and a colleague at Meals on Wheels that the defendant had threatened to kill her and grabbed her by the throat.
During a row on April 7, Ms Nibbs, who is originally from Kirkham, near Preston, Lancashire, had admitted seeing other men, taunting Nibbs by saying: “I have had sex eight times.”
The next day, the mother of five predicted her own killing as she left work, with the words: “If I’m not in Friday, I might be dead.”
On the night of Thursday, April 10, Nibbs attacked her in their Hoxton flat and knocked her out, Mr Aylett said.
He told jurors: “What might otherwise have been family tragedy now becomes terrible. You will, I am afraid, have to brace yourselves.
“Having attacked his wife, the defendant then took up a kitchen knife and cut off her head.
“Nor does the horror end there: having decapitated his wife, the defendant began to break her head into pieces with a mallet and a metal bar. He then flushed the pieces down the lavatory.”
The prosecutor went on: “Quite why the defendant decapitated Judith and then disposed of her head is not entirely clear but it may well be that he did it out of pure hatred at the sight of his wife’s face.”
After the killing, Nibbs wrote a suicide note addressed to his son Kirk, 30, and rang police to say they would find “a couple of dead bodies” at his home.
A paramedic was first on the scene but he was “fobbed off” by the defendant. A police officer then arrived and, seeing the headless corpse through the letter box, kicked the door in.
He found Nibbs in the bathroom with a shotgun in one hand and a kitchen knife in the other which he used to stab himself.
Mr Aylett said the officer acted with “remarkable bravery” in grabbing the gun from Nibbs and then taking the knife with the help of a fellow officer.
Nibbs was taken to hospital with stab wounds to the neck and stomach.
The defendant, who was also diagnosed with prostate cancer in 2012, was not well enough to be interviewed by police for a year.
Mr Aylett told jurors that the defendant admitted killing his partner but that he had been “defending himself”.
The defendant also said that he cut her head off because he thought she was a “snake” and disposed of it in the toilet, jurors were told.
But the prosecutor said there was “no evidence” that he suffers from mental illness.
The victim’s workmates described her as a happy, joyful, bubbly and open minded woman.
She had three daughters from a previous marriage as well as a son and daughter with Nibbs, having taken his name despite not being formally wed.
Her son Kirk says that since his mother no longer had to care for his younger sister, Nibbs felt she had changed and was not “the housewife” she used to be, the court heard.
A later examination of her computer showed that between 2013 and 2014 she had exchanged sexually explicit messages on Skype with a man called Khalid.
Nibbs, of the Charles Estate, in Hoxton, denies murder and obstructing the coroner from holding an inquest in the violent death of Judith Nibbs by disposing of her decapitated head.