Skydiver Stephen Hilder, who plunged 13,000ft to his death in sabotaged equipment, talked about committing the "perfect murder" on numerous occasions, an inquest heard yesterday.
Mr Hilder, 20, an Army cadet from Herefordshire, fell to his death at Hibaldstow Airfield, north Lincolnshire, in July 2003 while competing in the British Collegiate Championships after straps on both his main and reserve parachutes were cut.
His girlfriend, Ruth Woodhouse, told the Scunthorpe inquest that Mr Hilder was forensically aware and that they had numerous discussions about how to commit the perfect crime after watching the film Murder By Numbers.
Miss Woodhouse, who has a postgraduate degree in forensic engineering and science, said she had told him it just was not possible.
Following a ten-month murder investigation, Humberside Police said DNA samples taken from a pair of orange scissors found in Mr Hilder's car showed they had been used to cut the straps on his parachute and that only Mr Hilder's DNA was found on them.
Miss Woodhouse, 25, of Stafford, said: "If he was going to stage something then he wouldn't just chuck the scissors in the back of the boot."
The civil engineer said the couple had had "numerous" conversations about committing the perfect murder over a number of months. He had also shown interest in her course in forensics.
She said they had been in a "light-hearted, relaxed relationship" for about five months before his death.
When asked by coroner Stewart Atkinson whether she thought Mr Hilder may have committed suicide, she said: "There is no way, there is just no way."
She added: "I've looked back on the whole relationship since his death and even in hindsight I couldn't find a single thing to suggest that he would have killed himself, or why anyone would kill him.
"I don't understand why anyone would want to do that to Steve.
"I honestly don't believe he would do that to himself. I really don't think he would do it to skydiving.
"I don't think he would want to disgrace the sport like that."
The inquest heard that when Miss Woodhouse was questioned by police after Mr Hilder's death she joked that the only person who had a motive to kill him was herself because he annoyed her.
She said he teased her about her weight, but it was all done jokingly and he only ever teased the people he liked.
The inquest also heard that the couple were close to separating as their lives were going in different directions.
Mr Hilder, a cadet at the Royal Military College of Science in Shrivenham, near Swindon, Wiltshire, was considering joining the Royal Navy and Miss Woodhouse was about to move away to start a new job.
She said they had only joked about how things were coming to an end and had never talked about it explicitly. But they were still in a relationship at the time of Mr Hilder's death.
Describing Mr Hilder's passion for skydiving she told the coroner: "It was his life. Completely."
Laughing, she described how Mr Hilder used to show her his skydiving moves while standing on the bonnet of her car and being driven around.
He was happy with his own company, had more acquaintances than close friends and he wasn't someone who would just follow the crowd.
She added that he was "somewhat of a loner".
Mr Hilder had told her that she knew him better than anyone - and she had only known him six months.
Miss Woodhouse said Mr Hilder, who had debts of around #17,000, would spend any money he had on skydiving and enjoyment.
He had told her his biggest regret was failing his degree at Bristol University.
The hearing was adjourned until Monday.
