The unhinged homeless woman accused of stabbing a tourist changing her baby at Macy’s Herald Square was released from a psychiatric hospital hours before the attack — and bought the knife she used at the store after hearing “voices” in her head telling her to “kill,” prosecutors revealed.

Kerri Aherne, 43, had been let out of the Manhattan Psychiatric Center following a year of treatment when she strolled into the iconic Midtown store, purchased the blade and carried out the savage attack against a 38-year-old California mother Thursday afternoon, prosecutors said at her arraignment Friday night.

“She had purchased a knife at Macy’s and looked for someone to kill because voices in her head told her that she had to kill someone or else she would be killed,” Manhattan Assistant District Attorney Paul Barker told the court.

Kerri Aherne was arrested and charged with attempted murder for the alleged attack at Macy’s during the busy holiday season. James Keivom/Pool/ New York Post

Aherne was wearing a ragged white sweater and a face mask when she was arraigned in Manhattan Criminal Court on attempted murder, assault, child endangerment and weapons possession charges.

Barker noted that while the alleged knife-wielding fiend had no priors in the Big Apple, she was arrested in her native Massachusetts in 2018 for making online posts threatening to kill Sen. Elizabeth Warren and “getting a gun” to shoot someone at a “local police station.”

The troubled suspect first arrived in New York City via Uber from Massachusetts while on leave from a hospital there, he added.

It’s unclear why she then began receiving treatment at a separate hospital in Manhattan.

Aherne launched her vicious attack after seeing the victim enter the seventh-floor bathroom of the crowded flagship store with her 10-month-old daughter around 3:10 p.m., according to prosecutors and sources.

The victim, who had been holiday shopping at Macy’s with her husband and children, was repeatedly hacked in the back, shoulder, and arm with the knife.

“In the course of the assault, the baby fell from the table to the floor, but the mother was able to subdue the assailant,” Barker added.

Aherme was previously held at Manhattan Psychiatric Center following a year of treatment. James Keivom

The slashed mom, who works as a civilian for the Los Angeles County Sheriff’s Department, where her husband is a deputy, was rushed to Bellevue Hospital in stable condition, while her attacker was arrested.

While the victim’s back and arm were riddled with stab wounds, she was miraculously discharged from the hospital Friday.

“She is better,” her husband told The Post, declining to comment further.

Court records show Aherne has long struggled with mental illness.

The alleged knife-wielding maniac has no prior arrests in the Big Apple. Desheania Andrews for Ny Post

The Tewksbury native was deemed an incapacitated person in 2019, with her mother and sister appointed as her guardians, records show.

She was placed on a court-ordered treatment plan to keep her taking Prolixin, an antipsychotic medication used to treat schizophrenia, according to the records.

She last posted on her Facebook — which features anti-Trump comments, movie clips and incoherent rants — in 2018.

“Going a little insane,” one of her last posts prophetically states, along with a still from a Michael Jackson video.

Aherne was arrested in 2018 in Massachusetts for making online posts threatening to kill Sen. Elizabeth Warren and “getting a gun” to shoot someone at a “local police station.” AFP via Getty Images

Aherne’s attorney told the court he plans to seek bail at a future hearing.

But when the judge said she would be held without bail until her next court appearance, the clearly confused suspect interrupted, pleading not to be sent to another mental health facility.

“I don’t want to go to another hospital,” Aherne bemoaned to the court.

“They didn’t help me at Manhattan Psychiatric Center.”

She is due back in court on Dec. 17.