A social misfit who took mind-altering drugs made his 84-year-old grandmother a cup of tea before ferociously murdering her – slashing and stabbing her 50 times with a survival knife.

After Nathaniel Flynn killed retired head teacher Louisa Denby, he left his bloody handprint on the bedroom wall of the home they shared in Prospect Mount, Shipley, Bradford Crown Court heard yesterday.

Flynn, 27, a heavy cannabis user who experimented with mind-bending drugs, then attempted to murder nine-year-old Jason D’Arcy who was playing in the Carnegie skateboard park in Shipley after school.

He aimed a knife at the boy’s heart in a terrifying random attack before driving the blade right through his arm as the youngster tried to defend himself.

Jason suffered a divided artery and severed veins in his right forearm. He managed to get home where his father, Lee, a trained first-aider staunched the blood until paramedics arrived. He was taken by air ambulance to Leeds General Infirmary where surgeons performed a three and a half hour operation to save the use of his arm.

Jason told the police his attacker “went for my heart”. The blade struck his sternum bone, cutting his chest as he dodged the blows.

Flynn, who pleaded guilty to murdering Mrs Denby and attempting to murder Jason on July 1 this year, was yesterday jailed for life with an order that he must spend at least 24 years behind bars.

The court was told Flynn had been labelled a “violent psychotic” by a woman neighbour in the weeks before he murdered his grandmother. She said Mrs Denby was afraid of her grandson, who was weird and “chuntered to himself”.

Relatives had seen Flynn shouting and swearing at his grandmother and smoking cannabis.

Mrs Denby, who spoke several languages and loved music, had her grandson to stay in the school holidays when he lived in the US as a boy.

He was born in Bradford but moved to the States with his mother, Suzanne, when he was four.

Two years ago, Flynn came back to the UK and began living with Mrs Denby. He sold double glazing for a short time but largely depended on her for money.

The police were called to the house twice in February because of Flynn’s behaviour towards his grandmother but she defended him when relatives expressed concern.

Flynn was withdrawn from society and had a history of taking drugs. He began using cannabis at 15 and was convicted in the US of supplying cocaine. He said he had taken hallucinogenic drugs, including magic mushrooms, LSD and mescalin and had been a heroin addict.

In September last year, he broke his back falling off a cliff, possibly after taking drugs.

In the days before the murder, Flynn ordered survival equipment on the internet, including a carbon steel bushcraft knife, a camping knife, an SAS survival guide, a rope and a tarpaulin, the court heard.

Prosecutor Adrian Waterman QC said the purchases indicated that he planned to live hidden away somewhere.

After Flynn attacked Jason, he is believed to have holed up in a children’s den near Westfield Nursery in Sorrin Close, Idle, until he attacked a man in nearby Stanley Street, breaking his victim’s hand.

Shortly after midnight, armed police broke into the home Flynn shared with Mrs Denby and found her body in her bedroom.

She had 50 wounds to her head, neck, arms and torso and defence injuries on her hands.

Mr Waterman said the most significant injuries were a stab wound to her neck that severed her jugular vein and a knife wound to the heart.

Her injuries were consistent with a knife ordered by Flynn from Amazon. It was never recovered.

At 4am, armed police challenged Flynn on Westfield Lane, Idle. He was carrying garden shears and apprehended after an officer drew his pistol.

Mr Waterman said the only account Flynn gave of the murder of his grandmother was to a psychiatrist when he was in prison awaiting sentence.

He said he had not slept for two days. He was taking hypnotic drugs and thinking about killing Mrs Denby.

On the morning of the murder, he said: “I went and sat next to her and said I didn’t want to go on like that.

“I felt hopeless, I gave her a cup of tea and then attacked her. I cut her throat and I thought it would be fast and she would die quickly, like in the movies, but she moved and I sprang on her and kept stabbing.”

Flynn’s barrister, Jamie Hill QC, said his client had a problem functioning in society and suffered long-standing anxiety. He dealt with his problems by turning to drugs. He had a deep horror and sadness for his actions and did not expect forgiveness.

Mr Justice Keith sentenced Flynn to life imprisonment for murdering Mrs Denby. He must serve a minimum of 24 years in jail before he can even be considered for parole. He was sentenced to 14 years to run concurrently for attempting to murder Jason.

The judge told Flynn: “You say that you killed your grandmother – in effect as an act of mercy – because her mental functioning had deteriorated as she got older.

“No-one else seems to have noticed that, and a likelier explanation is that she just got on your nerves.”

He continued: “She trusted you by allowing you to live in her home, and you betrayed that trust by going into her bedroom and stabbing her to death.”

Mr Justice Keith added: “There is one other thing I want to say, and that is to express my own condolences to Mrs Denby’s family for their terrible loss, especially as her killer was her own grandson.

“I want them to know that I have taken into account the devastating effect which Mrs Denby’s death will have had on their lives.

“I also wish to pay tribute, as others more distinguished than me have already done, to Jason’s courage and to the efforts he has made to put the trauma of the attack behind him.”