He claimed former acquaintance Andre Gilling had never shown any remorse and was "proud" of his role in the killing.
Stanley Waipouri, 39, a homosexual, was bludgeoned to death in his flat in Rangitikei St, Palmerston North, on December 22, 2006.
He had severe head injuries, his ear was bitten off and penis mutilated during the attack. Arnopp and Gilling, then 17, were charged with his murder.
During their trial in December, Arnopp pleaded guilty and the jury returned a hung verdict on Gilling.
His retrial began this week and Arnopp - who was sentenced to 15 years' jail for the murder - was a key witness for the Crown.
Arnopp, now 21, had initially denied Gilling was involved to "protect" and "comfort" him.
But yesterday he said Gilling was the initiator as they punched, stomped and kicked Mr Waipouri to death.