A loner pie fortune heir told his friend he thought about killing him a couple of months before stabbed him to death in a frenzied attack, a court heard.
Dylan Thomas, 24, who is the heir to one of Britains biggest business empires, is accused of slitting his housemate William Bushs throat in a vicious attack on Christmas Eve last year.
A frightened Mr Bush, 23, had previously barricaded himself into his room as he slept after Thomas allegedly talked of killing him only two months before the fatal assault, the court was told.
Mr Bush, who was described as the accuseds only real friend, suffered 21 wounds to the neck after Thomas allegedly attacked him with two knives, severing his jugular vein.
Cardiff Crown Court was told that screams of horror could be heard from the street coming from the house during the seven-minute attack that night.
Thomas, the grandson of pie and property tycoon Sir Stanley Thomas, has admitted manslaughter but denies murder claiming diminished responsibility.
Dylan Thomas, 24, told his friend he thought about killing him a couple of months before stabbed him to death in a frenzied attack, a court heard
The court heard William Bush, 23, (pictured) had previously barricaded himself into his room as he slept after Thomas allegedly talked of killing him only two months before the fatal assault
The pair initially met as pupils at Christ College in Brecon and lived together in a property - which was owned by Thomas entrepreneur grandfather - in Llandaff, Cardiff.
The court heard how the young men were close, holidaying and golfing together, but the dynamics of their relationship began to change in the run up to Christmas.
Strong and athletic, Mr Bush, was a surveyor, who was about to move out of their shared house to set up a home with his girlfriend Ella Jeffries.
Mr Thomas was described as a loner and the weaker of the two men, with Mr Bush being his only real friend.
Prosecutor Greg Bull KC, prosecuting, said: William Bush was well liked, a quiet and passive man who shied away from confrontation.
He was looking forward to starting a new job and setting up home with his partner Ella.
Mr Bull said: Mr Thomas relied heavily on Mr Bush, but the dynamics of their relationship was changing. Mr Thomass attitude appeared to have changed.
Gregory Bull KC said it was Thomass case that he was suffering an abnormality of mind during the attack.
But the Crown argued that Thomas had planned the attack, having searched for the anatomy of the neck before messaging the victim: I need to see you, before you go.
The prosecutor told the court that Mr Bush told Ms Jeffries that one one occasion Thomas said to him: I thought about killing you, I just wanted to see what would happen if I do certain things.
Mr Bushs girlfriend told the police that the threat had been taken seriously by the deceased, who had barricaded his door while he slept.
Mr Bush was about to move out of the shared house where he lived with Thomas into a home with his girlfriend Ella Jeffries
Mr Bush suffered 21 wounds to the neck after Thomas (pictured) allegedly attacked him with two knives, severing his jugular vein
Screams of horror that could be heard from the street coming from the house during the attack, the court heard.
Thomas is accused of first using the flick knife to stab Mr Bush in the neck from behind, with the surveyor then trying to flee for his life.
Mr Bull added: It is clear that Dylan Thomas used the kitchen knife to stab Mr Bush in the chest and cut his throat severing the major artery in the neck, as a result of which Mr Bush bled to death.
A kitchen knife used in the attack - covered in blood, with its tip bent at some point during the incident - was shown to the jury, as well as the black flick knife.
Thomas told the police he stabbed Mr Bush in self-defence, insisting his friend had been having an episode.
Thomas sustained injuries to both hands during the incident, which he told the police happened while trying to disarm Mr Bush.
Mr Bull said the prosecution accepted that Thomas was mentally unwell and prior to the killing on Christmas Eve had begun suffering from psychosis.
Emergency services at the scene of the stabbing near to Llandaff Cathedral
Flowers left outside ofn the newly-built house in Cardiff where Dylan Thomas allegedly murdered his friend William Bush
He said: Following the killing, his condition worsened, as a result of which he has been receiving treatment for schizophrenia at the hospital.
The jury heard that Thomas had suffered at least one previous hallucination, in which he believed Mr Bush had threatened to drink his spinal fluid.
He said two psychiatrists had examined Thomas and differed in opinion about whether the act was caused by psychosis, with Mr Bull arguing the frenzied attack was intentional.
He said: The difference between the two doctors relates to what happened at the time of the killing.
Mr (Dilum) Jayawickrama is of the opinion that when you look at the evidence as a whole, it demonstrates that this defendant did know what he was doing.
It is the view of Mr Jayawickrama that even where a man is suffering from a psychotic illness, that does not necessarily mean that the psychosis has caused the act.
The trial continues.