The axe attack against Ms Guerin left her almost entirely decapitated.
Ms Guerin's son Thomas Guerin, 34, was also injured at the family's home, and Kane Love, 20, was attacked at a nearby property.
Both young men were hospitalised with serious injuries as a result of the attack.
Another woman, was also found injured nearby.
Today, the court was told how Mr Guerin and Nicholls had been best friends for 29 years.
Mr Guerin said he was "mentally destroyed" following the brutal death of his mother.
In his victim impact statement, which he read aloud to the court, Mr Guerin said he would never forgive Nicholls for murdering his mother.
"The hate I feel against Mana is very strong. When I see him in court that hate is aroused."
Nicholls had regarded Ms Guerin as a maternal figure, the court was told.
"It shocked me to the bone. My trust in people has diminished," Mr Guerin said.
Several other emotional impact statements were read to the court, with family and friends, whose names were suppressed, explaining how they had been "robbed of a wonderful loving person".
On the night of December 11, Nicholls drove to the home of Ms Guerin, which she shared with her son.
At this time, Nicholls was of the paranoid belief that people were out to get him, Justice Joseph Williams said.
Mr Guerin and Nicholls were in the garage of the home, when Nicholls asked for a glass of water.
"You drank the water and thought it tasted overly metallic," Justice Williams said. "In your paranoid state, you came to the conclusion that Thomas was trying to poison you."
Nicholls then struck Mr Guerin several times with a small orange and black tomahawk he was concealing in his jacket, causing injury to his side, head and shoulder.
"Sandra by this time had heard the commotion and was outside the garage."
Nicholls then punched Ms Guerin in the face, knocking her to the ground.
As Mr Guerin ran next door for help, Nicholls picked up an axe he found and struck Ms Guerin four times.
"[This] almost completely severed her head and she died quickly," Justice Williams said.
Nicholls then visited Mr Love's house. When Mr Love came outside, Nicholls swung the axe him.
When the axe broke, Nicholls used the handle to try to stab Mr Love, before he finally managed to throw Nicholls to the ground and escape inside.
As he walked away, Nicholls then assaulted a woman he passed on the street, before he was finally arrested.
At the time of the attack, Nicholls was suffering from a "paranoid psychosis" and had been for some time, Justice Williams said.
The fact Nicholls was known to have consumed illegal drugs on occasion may have been a contributing factor to the attack, he said.
He sentenced Nicholls to life in prison, with a minimum non-parole period of 14-and-a-half years for the attack he described as "savagely brutal".
"It is my hope... that you will see through and find insight about the appalling significance of what you did and own it," Justice Williams said.
"Sandra was an important maternal figure in your life."
Outside court today, Mr Guerin told reporters he was unhappy with the sentence and it should have been a "life for a life".
