David Leeman, 60, shot Jennie, 44, with a semi-automatic pistol at close range in a barn on the farm they once shared in rural Devon.
Their marriage was already on the rocks when Mrs Leeman, who had two sons and two daughters with the defendant during a 20-year relationship, started seeing Norman Laramy, 40.
Leeman discovered the couple’s affair six days before he killed Mrs Leeman, Exeter Crown Court heard. He said he heard voices in his head in the moments leading up to the death.
Leeman admitted manslaughter on the grounds of a “loss of control” because he was taking anti-depressants, which he said had an impact on his thought process.
Senior medical professionals were divided on whether the dosage Leeman was taking was sufficient to cause his behaviour on the day he shot his wife.
It took the jury nine and a half hours to acquit the property developer, of Parracombe, near Barnstaple, north Devon, of murder but guilty of manslaughter. Passing sentence, Mr Justice Butterfield said he was “wholly unimpressed” with the evidence that Leeman had been affected by the prescription medication he was taking at the time.
“This was a terrible tragedy. It is a tragedy for Jennie Leeman, whose life you took, and for her family.
“You became obsessed with the view that Mr Laramy was a paedophile and that your young daughter was going to live with your wife.
“It was false in the extreme but I am satisfied it was a view you genuinely held.”
The judge said Leeman had not lost his self-control when he took the handgun from its hiding place, cleaned it, loaded it and placed it in his pocket to meet his wife on the day he killed her.
Leeman showed no emotion as he was told he would serve a 12-year sentence for manslaughter to run concurrent with a five-year term for possessing the gun.