Collin Reeves knifed Stephen and Jennifer Chapple six times each at their house in Dragon Rise, Norton Fitzwarren, near Taunton in Somerset, on the evening of November 21 last year.
Reeves, also of Dragon Rise, had been involved in a row with the couple over designated parking on the new-build housing development since the previous May.
The 35-year-old former Royal Engineer used the ceremonial dagger given to him when he left the Army to kill the couple.
Reeves himself called the police just a few minutes after the killings to confess to what he had done, but later denied murder, claiming he was only guilty of manslaughter on diminished responsibility.
Collin Reeves in the dock
Elizabeth Cook
But two forensic psychiatrists found he was not suffering from psychosis or acute post-traumatic stress disorder, and diagnosed him with only mild to moderate depression.
A jury at Bristol Crown Court learned Reeves had also been having trouble in his own marriage, and less than an hour before he stabbed the Chapples his wife had asked for a trial separation.
Reeves was unanimously convicted of murder on Friday after a jury deliberated for five hours and 21 minutes.
On the night of the murders, Reeves was caught on a security camera climbing the fence separating his garden from the victims’ garden, and entering through the back door.
Jennifer and Stephen Chapple
Sam Malone/Avon and Somerset Police
A few seconds later Mrs Chapple can be heard screaming in terror, with Reeves shouting “die you f****** die”.
Mrs Chapple, 33, did not even have a chance to stand up from the sofa to defend herself while Mr Chapple, 36, was found close to the rear door.
The court heard the Chapples and Reeves previously had a good relationship but it had deteriorated when Mrs Chapple learned to drive and bought a second car.
Rows over parking spaces escalated to the point that both Reeves’ wife Kayley and Mrs Chapple had told their friends they were anxious about bumping into each other on the school run.
Ten days before the killings, Reeves was caught on a door bell camera approaching Mrs Chapple outside her house following an earlier exchange between her and Mrs Reeves.
Elizabeth Cook
He accuses Mrs Chapple of “f***ing gobbing off you cheeky little b****”.
The victim replies “she’s the one who started it, just f*** off”, to which he responds “what’s that you f****** c***, you fat bitch, you f****** … f****** c***”.
After the killings, Reeves was recorded in the background of the 999 call telling someone, believed to be his mother Lynn, “I couldn’t let her (or them) torment Kayley any more”.
Reeves said he had little memory of the incident but recalled sitting on the stairs in tears after the conversation with his wife.
He claimed he did not remember taking his dagger out of the picture frame in which it was usually displayed.
The defendant, who had previously recounted his fear of CCTV cameras and being under surveillance, said the next thing he recalled was a bright light coming on, and trying to get down on his front.
Reeves said: “I felt as though I had been seen or compromised, white light was a trigger when I was a soldier, when a light goes on or somebody sets off a flare, when that white light goes up something is going to happen."
Asked what else he remembered, the defendant said: “I had a feeling like it was me or them.”
Adam Feest QC, prosecuting, asked: “When your wife said you needed to have a separation, did you at least, in part, blame Jennifer because she had tormented (your wife)?” the prosecutor asked.
Mr Feest added: “I can’t let her or them ‘torment Kayley’ – I want to suggest that this is an accurate expression of why you went around to your neighbours that night. I’m going to suggest that’s the truth.”