KITCHENER, Ont. -- The woman serving a life sentence for the first-degree murder of Tori Stafford has added another conviction to her criminal record after admitting she beat up a fellow inmate and regretted not causing worse injuries.

Terri-Lynne McClintic, 22, was facing a charge of assault causing bodily harm for what the Crown called a "completely unprovoked" attack at Grand Valley Institution for Women, but instead pleaded guilty Wednesday to assault.

Crown attorney Julia Forward told court that the victim, Aimee McIntyre, was no longer co-operating with the Crown, so they couldn't prove certain bodily harm such as headaches she had earlier complained of having since the attack.

McClintic pleaded guilty in 2010 to the first-degree murder of the eight-year-old girl from Woodstock, Ont. Her former boyfriend Michael Rafferty was convicted earlier this year of first-degree murder, sexual assault causing bodily harm and kidnapping.

McClintic lured Tori away from her school on April 8, 2009, with the promise of seeing a dog.

McClintic shoved Tori in Rafferty's waiting car and the two of them drove about 130 kilometres north to a secluded field, where she was raped and brutally beaten to death.

She died from at least four blows to the head from a hammer and 16 of her ribs were broken or fractured.

Ontario Court Judge Colin Westman sentenced McClintic to six months on this assault, but noted it doesn't change her life sentence because sentences run concurrently in Canada.

Forward noted that the crime should be on her record should McClintic apply for parole under the faint-hope clause, which would come into effect in 2024.

McClintic's lawyer, Geoff Snow, said she had intended to plead not guilty and go to trial on the charge until the Crown disclosed that prison staff had intercepted a letter McClintic wrote to a friend on Jan. 30, the day of the assault.

In the profanity-laced letter McClintic recounts that she confronted McIntyre about something she had said about her, and detailed the assault, saying she "got in at least a couple shots, good ones, like one or two decent face shots."

Curled in the fetal position on the floor after several blows to the head, McClintic wrote that she kicked her repeatedly.

"Trying to get some shots through her arms, finally I brought my foot up tried stompin on her face a couple times," McClintic writes, ending the sentence with a smiley face.

McClintic had specifically requested McIntyre act as a mentor in a peer support program, and launched the assault "out of the blue" when McIntyre scratched her head, the Crown said. McClintic wrote that she could have done more damage -- McIntyre received a black eye and other bruises -- if she was in a larger room.

"Point made statement just not as loud as I would have liked it to be," McClintic writes.

The letter was addressed to the same person that she has sent many letters to in the past, writing about her fantasies of violence. Dozens of those were entered as evidence when she testified at Rafferty's trial.

Westman said McClintic must have some deep-seated anger issues and is "capable of doing a lot of damage."

He said he hoped McClintic would be able to learn how to live amongst her fellow human beings and be civil.

"It's tragic and one would hope that somehow you can come to some kind of peace," he said. "I can't imagine your turmoil you must feel within yourself."

At the court appearance McClintic wore a white sweater, jeans, white shower sandals and makeup, speaking only once to say, "Yeah."

Aimee McIntyre was also serving a life sentence for first-degree murder in the death of her former lover, though a new trial was recently ordered.

McIntyre's trial heard that she drove two men to her ex-boyfriend's apartment, one of them stabbed him six times, then McIntyre drove the pair away from the scene, helped them dispose of the knife and helped wash their clothes.

The two men pleaded guilty to second-degree murder. In a decision released in May the Court of Appeal for Ontario ruled that the trial judge made several errors that mean she did not receive a fair trial. The court set aside her first-degree murder conviction and ordered a new trial.