A "layabout" son killed his parents during a row about him being hungover, the Old Bailey has heard.
Daniel Dighton, 35, stabbed his father Barry, 61, four times and his 60-year-old mother Elizabeth more than 20 times at their south London home.
Mr Dighton was found in the loft of the house in Campden Road, south Croydon, after the attack in September 2009.
A plea of manslaughter caused by diminished responsibility by Mr Dighton was rejected. He denies murder.
Prosecutor Crispin Aylett, QC, told jurors the Dightons returned home at lunchtime from a shopping trip "to find their layabout of a son still in bed and hungover".
He said: "A quarrel broke out. A routine argument quickly got out of hand.
"The defendant armed himself with two knives, stabbing both of his parents to death in an attack of terrible ferocity."
After hearing screams neighbours alerted the police who found Mr Dighton hiding in the loft of the family home, the court heard.
Jurors were told the couple had both worked at Elmhurst School - Mr Dighton as headmaster and Mrs Dighton as head of year - until they retired in 2006.
Mr Aylett said they were described as happy and perfect companions but their only child was still living at home and struggled to make anything of himself.
In the past they had found him a flat and tried to get him a job but he spent his days lounging around the house and the evenings drinking in the pub.
Jurors heard Mr Dighton lied to friends about living with his parents, pretending he lived alone in the house and told "whopping lies".
Prosecutors have suggested that on the day Mr Dighton killed his parents, which he accepts, he "simply lost his temper".
The case continues.