He said he chose to try and knock his stepmother out before cutting her throat because he wanted to “kill her without distress, really”.
The killer then tried to post pictures of her blood-soaked body online, after trawling through similar gory images on a website.
Brathwaite, who admitted murder, was jailed for life with a minimum term of 15 years.
Judge Mr Justice Simon Picken said Brathwaite may have been lonely and unhappy at the time, but experts said he was not suffering from a mental health condition.
The judge said: “The savagery of the attack on this defenceless woman makes this case all the worse.
“No doubt inspired by the graphic images seen on the internet, you attempted to upload images of the attack.
Your obsession went to watching material including mass murders and massacres.”
Brathwaite had developed a sickening interest in beheading and mass murder videos on an “imageboard” website before committing the “horrific” murder.
Prosecutor Paul Hipkin said the teenager lived with his landscape gardener father, Charles Brathwaite, and his stepmother at their log cabin.
But Mr Hipkin said: “In the lead up he accessed images of killings of some brutality via a USA website. The images demonstrated murder, death and mutilation.”
On March 6, Brathwaite returned from school to his home at Broadmoor Farm, near Laugharne in Carmarthenshire.
He collected an axe while tending to his family’s livestock and a samurai sword.
Then he called to his stepmother to come outside to check on their cat.
Mr Hipkin explained that when Mrs Scourfield stepped out, Brathwaite struck her skull with the blunt side of the axe, before repeatedly striking her head as she lay on the floor.
He then used the samurai sword to slice and stab at her neck before unsuccessfully attempting to post the pictures online and then ringing 999.
In the emergency call, he said: “I’ve just killed someone. I know she’s dead. Her pulse has stopped.”
Asked why he had killed his stepmother, Brathwaite told the operator: “I’m just fed up with my life. I wanted to commit suicide but I couldn’t.
So I thought if I killed someone else it would get me out.”
He later admitted to officers: “I’m sure she loves me. I just wanted her dead so I could have a life.”
Paul Hobson, defending, said of the images Brathwaite viewed: “After time a natural revulsion to pain and crime became deadened to him.”
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