A 23-year-old Army private who said he gunned down a teenage friend’s mother and stepfather and a third person to free her from what he perceived as an abusive family situation was convicted Thursday of three counts of first-degree murder.

Joshua Charles Acosta, who was a private first class in the Army stationed in Fort Irwin at the time, told police he decided on his own to kill the mother and stepfather of then-17-year-old Kaitlynn Goodwill-Yost, who was dating his best friend and co-defendant, according to a recording of the law enforcement interrogation played for jurors. Co-defendant Frank Sato Felix, 27, of Sun Valley, wasn’t in on that part of the plan, according to Acosta’s confession.

Jurors deliberated for about a day before convicting Acosta of three murder counts and finding true a special circumstance allegation of committing multiple murders and a gun-use sentencing enhancement. Acosta, who is scheduled to be sentenced Dec. 14, faces life in prison without the possibility of parole.

Felix, who is due in court Friday for a pretrial hearing, would also face life in prison without the possibility of parole if convicted of having a role in the Sept. 23, 2016, killings of 39-year-old Jennifer Goodwill-Yost, her husband, 34-year-old Christopher Yost, and their friend, 28-year-old Arthur William Boucher.

Acosta told police that Felix and his girlfriend left her Fullerton home in the 400 block of South Gilbert Street before he “went to work.”

The shotgun-wielding Acosta killed Boucher, who was staying overnight as he would do occasionally, on the couch in the front room, then went to the master bedroom to kill Jennifer Goodwill-Yost in her bed, and then gunned down his third victim as Yost was attempting to get out the back door to the yard, said Senior Deputy District Attorney Troy Pino.

Kaitlynn’s two sisters, one 6 years old and the other 9, found the bodies in the morning.

Jurors heard the younger girl’s 911 call. At one point, the child tells the dispatcher, ” My dad is in the backyard dead and my mom is dead in bed …”

Police zeroed in quickly on the teen and her friends because she was missing when the bodies were found, Pino said.

At the time, he said, “they don’t know if Katilynn is kidnapped, dead or a part of this.”

When they found Acosta at Fort Irwin, he had three bullet casings in his shorts but initially “lied and denied” about his role as the gunman, the prosecutor said.

According to authorities, the three became friends through the “Furry” and “Brony” communities. The “furries” like to dress up as “anthropomorphic” animals in “mascot” costumes and “bronies” are adult men who are fans of the “My Little Pony” cartoons for children.

“There was a plan to emancipate (Kaitlynn) from her family because she was unhappy with her parents,” Pino said.

Kaitlynn Goodwill-Yost was granted “use immunity,” which means prosecutors cannot use her testimony against her in the case, according to the prosecutor, who said she has claimed “she had no idea her parents and Billy were going to be murdered.”

She has also claimed that Yost molested her and that her mother “physically abused her,” Pino said.

Acosta, who was an Army mechanic, bought bolt cutters and ear plugs on the night of the murders, according to Pino, who said the defendant told police he cut off a security cable on Felix’s father’s gun and used that weapon to commit the killings.

“That family is a twisted, wretched cesspool and it’s a festering wound,” Acosta told police. “I cauterized that festering wound.”

He further told police he wanted to “save” Kaitlynn’s sisters from “two monsters,” according to the interview with investigators.

“If we take Katie out of the picture, what stops him from going after the others,” Acosta said in the taped confession.

He said Boucher was “collateral” who “threw a monkey wrench” into the plans.

“I don’t really care if I go to prison or if my life ends here,” Acosta told police. “All I care is this little girl is going to be safe and sound.”

Acosta’s attorney, Adam Vining, wanted jurors to also consider the legal theory of imperfect self-defense of others, which would have allowed them to consider voluntary manslaughter, but Orange County Superior Court Judge John Conley declined. Vining had argued that because of his client’s autism, he felt he was acting in defense of his friend and her younger siblings, so if jurors agreed they would have had to acquit the defendant.

Vining said his client was a “bronie” who had a “My Little Pony” doll made for him by a friend stuffed in his backpack when he went out to do combat exercises. Vining said it’s common for people with his client’s autism disorder to congregate with “furries” and “bronies.”

Jennifer Goodwill-Yost was a “furry,” who would bring her daughter to gatherings, according to Vining, who said Acosta’s autism is “central” to understanding the defendant’s actions.

“Autistic people tend to be naive, emotionally immature,” Vining said in his opening statement. “They tend toward black-and-white thinking.”

Acosta was born to a mother afflicted with fetal alcohol syndrome, who gave birth to two children with methamphetamine in their system, Vining said. Acosta and two of his sisters were adopted by a Los Angeles police detective and his wife, Vining said.

Acosta’s mother got out of jail just a day before she gave birth to him, according to the defense lawyer, who said Acosta’s sister was molested at the age of 9 by a teacher who was not prosecuted.

When his investigators went looking for Acosta’s childhood friends, they couldn’t find any, he said.

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