The incident took place in Papakura in November last year when the victim invited Herkt and other around to celebrate a successful inspection on his Porchester Rd unit.
The group managed to persuade a local liquor-store owner to give them a box of Cody's RTDs on credit but while sitting down to drink them, an argument erupted.
The Crown said it was unclear why but a fist fight took place, possibly because Herkt stole some of the cans.
He was forced out of the flat, bleeding from his face, and persuaded a neighbour to give him a cricket bat and a kitchen knife before returning to Mr Greenslade's residence.
"The defendant was angry, armed himself with a potentially fatal weapon, kicked down the back door and launched himself at the deceased. He killed him and he meant to do so," prosecutor Gareth Kayes said.
The victim sustained stab wounds to the back of the head, base of the neck and a seven-centimetre cut along the scalp, among others.
But Justice Moore said the fatal wound, up to 8cm deep, was one that cut the liver in two places, nicked the ribs in three and pierced the lower parts of the victim's heart.
"There was no spurting blood," the judge said. "[The heart] slowly but steadily bled with each pumping motion."
Mr Greenslade was conscious on the drive to Papakura accident and emergency but collapsed outside the clinic where he could not be resuscitated.
Herkt's lawyer Peter Kaye said his client was being medicated for paranoid schizophrenia at the time but his alcohol intake would have reduced the effectiveness of the medication, resulting in "disordered thought processes and paranoia".
The jury rejected that explanation.
Justice Moore said the motive behind the attack was "trivial in the extreme".
He convicted Herkt and gave him a first-strike warning before expressing his gratitude - and that of the victim's family - for the diligent way the jurors took to their task.
Sentencing will take place next month.
