Following a national manhunt, Martinson, 18, was arrested in Indiana with her boyfriend, Ryan Sisco, then 22, on March 8, 2015, a day after the murders. She was initially charged with two counts of first-degree intentional homicide for the murders of her stepfather, Thomas Ayers, 37, and her mother, Jennifer Ayers, 40.
Martinson had pleaded not guilty by reason of mental disease or defect last June to those charges, which carried with them a maximum sentence of life in prison. Since then, her attorneys and prosecutors struck a deal which resulted in Friday's plea.
As part of the plea deal, prosecutors will recommend a sentence of forty years in prison. Her lawyers will recommend an eight-year sentence.
New Court Records: Defense Claims Martinson Was Physically and Sexually AbusedMartinson told authorities that her stepfather physically abused her mother for years and was mentally and verbally abusive to her.
In the plea agreement, the State of Wisconsin conceded that "the homicides ... were caused under the influence of adequate provocation," according to court records filed by the defense Friday along with her plea.
The new court filings also reveal that two days before the murders, on March 6, 2015, which was Martinson's 17th birthday, she sent Sisco a Facebook message saying, "I woke up this morning to my step dad beating my mom ... I can't take [it] anymore, he's gonna kill her if she doesn't leave soon and I don’t want to be around w[h]en that happens," the court records state.
Saying that she thought her mother was going to leave her stepfather, she continued, "I [expletive] hate them, too. I want to kill him so [expletive] bad, just take one of his guns and blow his [expletive] brains out."
According to the new filings, experts who interviewed Martinson after her March 9, 2015, arrest, say that she suffers from severe depression and post-traumatic stress disorder after allegedly being physically and verbally abused by several of her mother's boyfriends and raped at the age of nine by one of them.
She was further traumatized by allegedly witnessing her stepfather abuse her mother, step sisters and half sister and making her and her siblings watch him kill baby animals in front of their parents to see the reaction of the animal parents, according to the filings.
She is scheduled to return to court June 17 for sentencing.
"People didn’t know about the abuse she went through," Martinson's neighbor, Roy Rasmussen, tells PEOPLE. "This is a tragedy for everybody involved."
But Ayers's brother, Don Ayers, is unhappy with the plea agreement. "The day before the murders, she wrote on Facebook that she wanted to kill them," he tells PEOPLE. "To me, that's premeditated. They should have left the charges at first-degree murder."
He is also upset about how his late brother is being portrayed as an abuser. "Thomas and Jennifer are being judged right now by what she is saying about them, but they aren't here to defend themselves because she killed them. I think she stretched the truth to save her own neck."
What Led a 'Happy' Teen to Kill?When word of the murders spread, family, friends and residents of Piehl, Wisconsin, were left asking, "Why?"
A friend of Martinson's from Rhinelander High School described her friend to PEOPLE last year as a quiet, kind and happy teenager who "put everyone else before herself" when she first moved to the small, rural town in 2014. Martinson often babysat for her younger sisters and held down a job at a local store so she could pay her bills, her friend said.
But her outward nature belied something darker. Soon after her arrest, police learned that Martinson was a self-described "horror fanatic" who had penned a blog she named "Nightmare," in which she wrote graphic stories and poems about blood, mutilation, death and dying. She also posted macabre pictures on her Pinterest page, on which she said she came from "the dark, haunted woods of Wisconsin."
Another friend of Martinson's told PEOPLE, "This was a cry for help, but nobody was listening."
Indeed, the court records filed Friday paint a disturbing portrait of her home life and the abuse she allegedly suffered at the hands of her mother's previous boyfriends, including when one of them beat her, burned her with a cigarette and raped her when she was nine years old in a vehicle while his friend and neighbor watched.
After her mother met Ayers on an online dating site, the two left their home in Kansas to live with him in North Dakota. Her mother married Ayers in December 2011 after knowing him for nine months, the court records state. In 2013, they had a child together – Ayers's seventh.
Ayers had a criminal record, including numerous prior arrests and convictions starting in 1998 including driving under the influence, domestic battery, assault, menacing and kidnapping. A former girlfriend, who began a relationship with Ayers at 16, told authorities she believes he suffered from bipolar disorder.
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Martinson said Ayers was physically abusive to her mother. The new filing said he would "push her, smack her choke her and hold a gun to her head" if "something was not cooked correctly, something was not cleaned or she did not rub his back effectively.
"Conflict in the Ayers' family home," according to the court records, led Martinson in 2013, to move back to Kansas to live with her father, Jeremy Martinson. She moved back to North Dakota after just two weeks, saying her father allegedly abused her physically, the court records state.
On Dec. 13, 2013, Kansas police were called to her father's house after a report of a "physical altercation," when her father allegedly "admitted to slapping his daughter," the court records state. She reported other incidents of physical abuse, including "punching, shoving and kicking," according to the court records.
Martinson's father stopped speaking to her after the Dec. 2013 incident and has allegedly said that while he knows she has been incarcerated, "he has no intent to contact or assist her let alone act as her guardian for medical purposes," the court records state.
"He expressed only concern about media reports of the case that would cast him in an unfavorable light," the court records state," the records add.
Attempts to reach Jeremy Martinson were unsuccessful.
The court records state that Martinson's mother did not want her to move back with them, but "Thomas Ayers did."
New Life in Wisconsin Ends in MurderIn 2014, Martinson moved with Ayers and her mother and siblings to Oneida County, Wisconsin. The family lived on a large parcel of land so Ayers could hunt.
Police who investigated the murders observed that the home had "an arsenal of loaded, unlocked and easily accessible firearms and ammunition, despite Thomas Ayers's legal prohibition from possessing those items and the presence of children in the home."
While living in Wisconsin, Martinson would often allegedly "wake up to her mother screaming and begging for her life," the court records state.
Two of the other children in the home told police how Ayers abused Martinson's mother and would hit them "hard" with a thick belt "until their buttocks nearly blistered," the court records state. They told police that their mother hit them with a belt, too. One of the children told police she would regularly feed, diaper and bathe their youngest sister because their mother did not do "a lot of motherly stuff."
Two of the children told police that Ayers choked one of their dogs and threw it against a wall "for not listening" before he shot and killed it and fed it to a bear. He threatened to shoot their other dog when one of the younger children failed to listen to him, the court records state.
Martinson told authorities that Ayers was "fond of killing baby animals in front of their parents," and would instruct her and her sisters to "watch the reaction of the parent animals to the death[s]."
Martinson told authorities that Ayers never physically or sexually abused her, but said he once knocked down her bedroom door, injuring her nose, court records state.
But Martinson told authorities about "consistent mental and verbal abuse coming from Thomas Ayers" involving strict house rules.
He allegedly required her to wake up at 4 a.m. on weekdays and 6 a.m. on weekends to complete a list of chores. He allegedly restricted her use of the computer and did not allow her to have visitors at their home. She was allegedly only allowed to go out once a week and had to give her earnings from jobs to him, which were allegedly "dispersed according to Thomas Ayers's directives," according to the court records.
She says that shortly before the murders, on Feb. 28, 2015, he called her an "ignorant bitch" and said she "would fail in life and amount to nothing," the court records state.
The Day of the MurdersOn March 7, 2015, the day of the slayings, Ayers and Martinson's mother allegedly confronted her over her relationship with Sisco. They told her she could no longer communicate with him and took her keys and phone, court records state.
While Martinson's mother said she should leave, Ayers allegedly said she should be home schooled and "essentially be placed on house arrest for the foreseeable future," according to the court records.
Martinson left the home and went to a neighbor's house on foot, but Ayers allegedly followed her in his truck and told her to get in so they could return home.
She told authorities she went to her bedroom and "armed herself with one of the many loaded shotguns in the house," according to the court records, "for the purposes of killing herself."
Ayers stormed up to her room after telling his wife, "She's probably doing something stupid."
When he began banging on her door, Martinson said she "considered whether Thomas Ayers should die rather than she," the court records state.
She fired the first shot to his neck. The second was a contact wound to his temple, "fired to ensure that he was dead and could not hurt her," according to the court records.
Martinson said she sought comfort from her mother for what she had done, but her mother tended to Ayers and yelled at her for shooting him. Her mother allegedly "then armed herself with a knife and approached" Martinson. The teen wrestled the knife away from her mother and stabbed her more than 30 times "with considerable force," the court records state.
Martinson, say her lawyers in the documents, "acted upon provocation…completely losing control at the time of the commission of the homicides, demonstrating anger, rage and exasperation as a person of ordinary intelligence and prudence under similar circumstances would have done."