Clayton Weatherston will not appeal against his life sentence with a non-parole period of 18 years, his family says.
The murderer of 22-year-old student Sophie Elliott sparked public revulsion for his apparent lack of regret on the witness stand.
Now his sister Angela Muir, Now, who has visited him in Christchurch Men's Prison, says he feels deeply remorseful.
"I believe he felt that all along," she told the Herald on Sunday. "He is not a monster and he is not a killer in the true sense of the word. He might have done it but he is not a murderer in the true sense - he was defending himself."
She added: "I am sure he has a lot of feeling that he doesn't want to discuss."
The jury rejected Weatherston's claims that Elliott came at him with a pair of scissors on 9 January, 2008.
They found him guilty of murder for stabbing her 216 times in her own bedroom and mutilating her, while her mother, Lesley Elliott, tried to break down the locked door. Lesley Elliott is now campaigning for victims' rights.
Angela Muir said her brother would not appeal against his sentence.
He remained in isolation by choice, she said, but eventually prison management would want to shift him in with other inmates.
Other prisoners have reportedly dubbed the 33-year-old "Chucky", after the maniacal knife-wielding doll in the horror movie Child's Play.
But Muir rejected claims that her brother had asked other inmates to call him "Dr" Weatherston.
"I am sure he will make friends because he talks to anybody." Most people were in no position to judge him, she said. "They are not facing this from a personal experience."