A man who killed his mother — nearly decapitating her — will be eligible for release from jail in 11 years' time, prompting outrage from his grandmother.

Bradley Craig Mottram was 19 years old when he murdered his mother Simone Mottram, 51, in the north-western NSW town of Inverell in 2015.

At a sentencing hearing on Thursday, Acting Justice Peter Hidden described it as a "dreadful crime committed by a very troubled young man".

"Mottram punched [his mother] in the face several times [then] hit her over the head with a bourbon bottle," Justice Hidden said during sentencing at the Supreme Court on Thursday.

"He obtained a carving knife from the kitchen and returned to … inflict a large number of other stab wounds … causing an almost complete transection of the neck."

The court heard Mottram was a talented student, but was described as "neglected and abused" by his mother over a long period.

"He was constantly exposed to men with whom the deceased had formed relationships after knowing them only briefly," Justice Hidden said.

"He spent time with her at their houses, where she would drink alcohol, use drugs, and on occasions engage in sexual activity in his presence."

An apprehended domestic violence order was taken out by Mottram against his mother, which she breached on several occasions.

Discount on sentence for early guilty plea

When Mottram was arrested on October 19 the court heard he was in a distressed state.

He admitted killing his mother and voiced concerns about being killed by bikies who had taught him to "cook" ice.

In a note entitled My Last Words he wrote:

"This was not planned. I was drunk, it was an accident. She just, she wired me from birth, it was an accident, I'm sorry."

Mottram received a discount on his sentence because of his early guilty plea.

His was sentenced to 18 years with a non-parole period of 13-and-a-half years.

Mottram's grandmother, Maureen O'Bryan, described the sentence as "totally inadequate".

"Do you want that guy out on the street in 11 years' time doing the same thing when he takes his drugs?" she said.

"I'm angry. That's what keeps me sane. We know [my daughter] had problems, that is irrelevant.

"He knocked her unconscious, left the room, went and got a knife and came back.

"He could have left the room when she was unconscious."

Mottram will be eligible for release on parole on April 19, 2029.