Four months after the brutal murder of Poppy Devey Waterhouse, the 25-year-old is still 'wrestling with with the conundrum' of why he did it. Leeds Crown Court heard.
Leeds Crown Court heard the breakdown of the couple's relationship had led him to behave like a "Jekyll and Hyde character" in the weeks leading up to the attack.
Towards the end of November, Atkinson flew into a jealous rage and attacked Miss Devey Waterhouse's new boyfriend after see them out together.
The prosecution described how the attack "was a hint of his brewing jealousy."
But at other times friends had described how he had seemed to have accepted the break-up.
Then, in the early hours of December 14 after a night out drinking with colleagues, Atkinson carried out the "inexplicable" and "harrowing" knife attack on his partner of three years.
He had been to his work's Christmas night out earlier in the evening.
He was seen leaving Call Lane Social in the city centre at 2.20am, stopped for food on the way home, and walked back to the flat at 3am.
Poppy had spent the evening at home alone.
She had spent some time talking to her new boyfriend on the phone and gone to bed around 10pm.
Atkinson was captured driving his car later that morning at 5.40am in the Oulton area of Leeds.
He dialled 999 three hours later at 8.40am.
Soon after, paramedics were let into the flat at Saxton Apartments where the found Miss Devey Waterhouse's lifeless body in a pool of blood in the hallway.
Prosecutor Jason Pitter QC said: "What happened in the interim? He had armed himself with a kitchen knife and murdered Poppy in an unprovoked attack fuelled by his raging jealousy.
"There were no witnesses to the attack other than the defendant."
There was no evidence that Atkinson, a maths graduate with a First Class honours, had ever been violent to anyone before.
When he finally confessed to the killing he told police: "I did not know how many times I stabbed her. I just didn't stop.
"I only stopped when it all went calm and she stopped moving."
Atkinson was assessed by a forensic psychiatrist after the killing but the precise reasons for his behaviour are still unclear.
The court heard that the attack may have been triggered by jealousy and the breakdown of his relationship with Miss Devey Waterhouse, leading to a "sense of loss of control and low self esteem which engendered powerful feelings of anger".
Atkinson's barrister, Richard Wright QC said it was "quite inexplicable to any observer" how an otherwise good-natured and high-achieving person could commit such a grave crime.
He added: "That is a conundrum Joe Atkinson will have to wrestle with for the rest of his life.
"He can do no more than what he now has done and admit this offence, put aside the lies that he has told, say sorry and hope that those whose lives have been utterly wrenched apart by his behaviour will find some peace."