He sent the woman $13,500 over the ensuing months, $2000 of which he had borrowed from his sick grandmother.
Justice Venning said the mother of two used the cash to fuel her methamphetamine and gambling habits.
Edmonds "obsession" was apparent through his phone and text messages, the judge said, which she rarely returned.
By November last year she had started a relationship with Mr Bettink, unknown to the defendant who drove down from Wanganui to see her on a whim.
When he found Mr Bettink at the house the pair spoke over coffee but tempers exploded when the woman in the centre of the love triangle returned home.
After arguing, Edmonds pushed her to the ground, fracturing her jaw.
Mr Bettink intervened but Justice Venning said no witnesses corroborated the defendant's story of being on the receiving end of a punch.
Edmonds launched a vicious attack.
He punched the victim to the floor and continued to unleash blows to his face as he lay there.
In interviews with police, Edmonds told them there were only two or three blows at about half strength but pathologist Dr Fintan Garavan said there were likely many more at much greater force.
"Blood spattered from the punches was on a number of objects in the vicinity," Justice Venning said.
Defence lawyer Mina Wharepouri said his client was truly remorseful for his actions, which would not have come about had it not been for the woman's manipulation.
"His life was very much put on a collision course by [her] and her greed which saw her syphon Mr Edmonds of his time and energy and finances to fuel her gambling and drug addiction," he said.
Members of Mr Bettink's family were also keen to express their dislike for the woman in their victim impact statements but they reserved the most stinging remarks for the killer.
"Only you know what really took place but have chosen not to reveal the truth," his wife Linda Bettink wrote.
"He of all people didn't deserve to have his life cut short in that violent way. You didn't even know him."
Her father was also keen to speak out. "[Edmonds] has to live with his lies for the rest of his life. May he live in hell and then die," he said.
Justice Venning jailed Edmonds for six years four months but refused the Crown's application to impose a minimum non-parole period.