Paul McManus left Liam Scott with a “gaping wound” in his neck following the attack at HMP Shotts on March 22, 2023.
Lifer McManus was there serving a minimum of 26 years for a killing that shocked the country in 2014.
The then 19-year-old stabbed Isabelle Sanders, 51, a total of 37 times during a robbery at her home in Crookston, Glasgow.
He had initially turned the knife on her 86-year-old partner – retired policeman Norman Busby.
McManus – who already had a string of convictions – was found guilty of murder, attempted murder and two other robbery-related charges.
McManus, now 29, was back at the High Court in Glasgow this week.
He admitted assaulting Scott to his severe injury, permanent disfigurement and to the danger of his life.
McManus was handed a five-year sentence – but it will begin from the date of his guilty plea meaning his 26-year term was not extended.
Prosecutor Chris McKenna told how Scott was sitting on the edge of a snooker table in a communal area of Shotts when he was confronted by McManus.
A row broke out and Scott shoved him away.
Mr McKenna then explained: “After a short time, McManus raised his right hand and struck Scott in a downward motion to the right-hand side of his neck with a weapon.”
McManus then casually walked off back to a cell.
Prison staff were alerted to the attack, found an injured Scott and he was taken to hospital.
He had a three-inch gaping wound to his neck exposing the muscle.
Mr McKenna said he has been left scarred for life. Scott stated he had been in an “altercation” with McManus and had been struck with a razor blade.
Ross Yuill, defending, told the hearing McManus’s earliest current release date is in 2040 when he will be aged 46.
McManus had got a valued job as a “pass man” in Shotts but lost it after this attack.
Solicitor advocate Mr Yuill added: “He was kept in effectively a solitary environment for two months.
“His risk was returned to high as a result of this incident.”
The lawyer said this latest conviction could have an effect on when he is eventually freed.
Mr Yuill: “It is my submission on the basis of the length of the life sentence and his age when he is released, it is open to the court to make any sentence concurrent.”
Lord Arthurson ordered the five years – cut from seven due to the guilty plea – to run from the date of the hearing.
