Trimaan "Harry" Dhillon was reported by Alice Ruggles, 24, weeks prior to her death after he became obsessive when she broke off their relationship.
Alice was the target of relentless messages and calls from the lance corporal, who also showered her with gifts she didn't ask for.
He also threatened to kill himself and got in touch with his friends and family, the Sun reports.
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But then, on On October 12, 2016, he snapped. Alice’s throat was slit at least six times by Dhillon, who had climbed into her Gateshead home via an open window.
Now a new documentary, Faking It, has shone some light on how his eventual expression of his sick actions went down.
Asked by an officer whether he killed Alice, video footage from his police interview shows him say: “No I did not murder Alice".
Dr Cliff Lansley, however, then gives insight into the giveaway actions made by Dhillon that prove he is lying.
"Early on in the police interview, he gets hit with a silver bullet. We get this little contraction of his buccinator muscle on his left cheek where his left cheek and the left lip rises slightly,” he said.
"It's a very tiny microexpression of contempt.
"Context is everything. We've got a female officer so is he contemptuous towards women? Or is he contemptuous and confronted by the question, 'did you murder Alice?'
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"He avoids a contraction. A contraction is a way of describing the 'do not' combination. If he was telling the truth then often it would be contracted to I didn't kill her."
He also added that the shake of Dhillon’s head to "reinforce" his point is another telltale that he actually did.
"He does a head shake 'no' that reinforces the did not.
"If the head shake gesture continues when the words are finished, the person is in convince mode.”
He added: "The fourth indicator that tells us it's a lie is the eye closure. If we look closely, the eyes close for about a third of a second.
"Now this is not a blink, while he's shaking his head his eyes are closed… he's distancing himself from the statement.
"We know of four indicators that when you put them together, tell us that his denial here is a confession."
In August 2016, Alice is understood to have split with Dhillon after she learned he had been speaking to other women online.
His stalking of her got really bad when he learned that she was beginning to see someone else.
In the six weeks before she died, he called her 290 times.
Dhillon was sentenced to life with a minimum of 22 years for the act of "utter barbarism".
You can watch Faking It on Quest Red at 10pm on Saturdays or on discovery+.
If you’ve been affected by this article please contact the National Stalking Helpline on 0808 802 0300 or visit www.alicerugglestrust.org
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