Screams of horror could be heard as a man killed his friend at their home in Wales on Christmas Eve, a court has heard.

Dylan Thomas, 24, is accused of murdering William Bush, 23, in a “frenzied attack” on December 24 last year in Cardiff.

Thomas, previously of Llandaff, admitted manslaughter at a hearing last week but denies murder.

He appeared before Cardiff Crown Court by video link from a psychiatric hospital on Wednesday, wearing a grey jumper and trousers, with close-cut black hair.

Gregory Bull KC, speaking on behalf of the prosecution, said it was Thomas’s case that he was “suffering an abnormality of mind” during the attack.

But the Crown argued that this was not the case and that Thomas had planned the attack.

He said the defendant had told his friend a month or two before the incident: “I thought about killing you, I just wanted to see what would happen if I do certain things.”

Mr Bush’s girlfriend told the police that the threat had been taken seriously by the deceased, who had barricaded his door while he slept.

Mr Bull described events leading up to the incident to the jury of 10 men and two women, with “screams of horror” that could be heard from the street coming from the house during the attack.

It is the prosecution case that Mr Thomas made his way into the property. He entered through the kitchen where he armed himself with a large kitchen knife and a black lock knife or flick knife. He then proceeded to go up the two staircases and into the room of William Bush

The prosecutor told the court Thomas was driven to the house he shared with Mr Bush in Llandaff by his grandmother on Christmas Eve, telling her he wanted to walk his dog.

He said: “The next thing she knew was that a few minutes later, Mr Thomas was banging on her window plainly distressed.

“Screams described as screams of horror were heard by (a man) who was in the street.

“It is the prosecution case that Mr Thomas made his way into the property.

“He entered through the kitchen where he armed himself with a large kitchen knife and a black lock knife or flick knife.

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

“He then proceeded to go up the two staircases and into the room of William Bush.”

Thomas is accused of first using the flick knife to stab Mr Bush in the neck from behind, with the latter 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, 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”.

Leading up to the attack, the court was told Thomas searched on his computer for the anatomy of the neck and messaged Mr Bush telling him: “I need to see you before you go.”

Thomas sustained injuries to both hands during the incident, which he told the police happened while trying to disarm Mr Bush.

A video shown to the jury showed a police officer confronting Thomas on December 24, covered in blood, his hands out in front of him.

The police officer asked him, “where’s the person who did this?” and he replied, “I think he is dead, I f****** disarmed him”.

The two men had been friends since they met at school at Christ College Brecon.

Mr Bull described Thomas as having few friends and was “something of a loner”, with his grandmother calling him a “quiet person”.

Thomas is expected to defend himself against the charge on the grounds of diminished responsibility.

Mr Bull said the prosecution accepted that Thomas was “mentally unwell” and prior to the killing on Christmas Eve had begun suffering from psychosis.

It is the view of (psychiatrist) Mr (Dilum) Jayawickrama that even where a man is suffering from a psychotic illness, that does not necessarily mean that the psychosis has caused the act

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.