He described how Holl, who was living with his mother in Picton at the time, grabbed her, dragged her into the lounge and struck her several times in the back of the head with the blunt end of an axe.
She was then dragged into the bathroom where Holl struck her in the face and chest with the axe until he was sure she was dead.
Ms Grace died of multiple blunt force injuries, including a laceration to her heart. She had received 35 wounds.
Holl was arrested at the scene soon afterwards by police, who had been alerted by neighbours.
Justice Collins said the murder was a calculated and deliberate "revenge plan" which included assembling weapons and equipment to be used in the crime.
It involved a high level of brutality, involving multiple blows with an axe, he said.
A starting point for 18 and a half years minimum imprisonment was reduced for Holl's guilty plea, his youth at the time of offending, and the contribution of his personality disorder.
Justice Collins said Holl did not have a psychotic illness but suffered from a severe personality disorder, which "probably contributed in some way towards your violent behaviour".
Holl was sentenced to life in prison with a minimum non-parole period of 15 years, and given a three strikes warning.
