"None of us will ever know precisely what was going through Jordan's mind when he decided to shoot Ms Kurth. Jordan professes not to recall,'' Justice Heath said.
"Something happened within his brain that made him shoot and, as I have said, we will never really know what that was.''
Mr Lock read out a victim impact statement in court today, saying: "When I returned from crossing that river on that day and saw the two bloody drag marks leading into the spare room, seeing Rose laying on the floor ... I thought my world had ended.''
Ms Kurth's daughter, Lisa-Ann Raymond told Nelson she had not been able to grieve.
``Nor listen to what you did to my mum on that day. I've heard it once and it's a sickening pain.''
Justice Heath said he did not think society's interests would be enhanced by sentencing a 13-year-old boy to life imprisonment.
``I acknowledge that a period of 18 years imprisonment is also likely to be seen as crushing by someone of Jordan's age. But, at least, there is a finite term to be served.''
Peter Williams QC said the sentence seemed ``pretty harsh and lengthy''.
He said the only hope Nelson had of leading a productive life after being released was if he was treated humanely while behind bars.
``It's a pretty wretched situation. I would hope that there is a future for him where his relative infancy is taken into account and that he's given a chance to develop a reasonable type of personality and possibly also to learn skills that will enable him to find work when he's ultimately released.''
Mr Williams pointed to the 1954 Juliette Hulme and Pauline Parker case when the girls, aged 15 and 16, murdered Pauline's mother.
``They were both young but they both went on to pursue useful careers.''
Child, Youth and Family acting general manager of residential and high needs services Nova Salomen said Nelson would begin his sentence in a secure youth justice residence.
He would stay there at least until his 16th birthday.
The residence is a standalone facility, not attached to an adult prison. It has its own school for residents and other facilities to support young people.
Stephen Bell, who runs youth support service Youthline, said there had to be a consequence for Nelson's ``devastating'' action.
However, his treatment in jail would dictate whether he would leave as a better person.
Mr Bell said the entire situation was a tragedy.
``For him and certainly for the person killed and their family.''
