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Brandon Horst was fast asleep at his home on St. Paul's West Side on Aug. 5 when Aaron W. Allen slipped into his bedroom and fired a single shot to the National Guardsman's head, killing him instantly.

A remorseful Allen publicly confessed Wednesday morning in Ramsey County District Court, telling a judge and Horst's family that Horst's wife was to blame for concocting the murder plan and that "things need to be set right ... I feel that his family deserves justice."

Allen, 25, of South St. Paul, said that he shot Horst because he believed that Horst had abused his wife, causing her to miscarry three times.

As part of the plea deal, he agreed to plead guilty to second-degree intentional murder and a 40-year prison term. He will be formally sentenced April 22.

Both defense and prosecution attorneys cast Allen as a vulnerable man manipulated by Heather Horst into a 6-foot-4, 360-pound weapon.

Allen admitted as much, testifying Wednesday that Heather Horst, 25, knew that he was physically and sexually abused as a child. Allen testified that he is bipolar and has other mental health issues, including post-traumatic stress disorder and borderline personality disorder.

Allen said Heather Horst's allegations of abuse affected him deeply because of his past, saying that he witnessed his mother being abused and thrown down stairs when he was a child. Heather Horst levied the same allegations against her husband and said he had punched her abdomen, causing her to miscarry.

Assistant Ramsey County Attorney Karen Kugler said in court Wednesday that Heather Horst was never pregnant and never miscarried.

Allen was engaged to Brandon Horst's stepsister at the time of the killing. There is now a no-contact order between the two.

Heather Horst began making allegations of abuse against her husband in June, although she never had any marks and provided "very little detail," Allen said.

Allen testified that he repeatedly urged Heather Horst to leave her husband and call police. But, he said, she told him she couldn't leave because she would lose everything and would be disowned by her family. She said police wouldn't act on her allegations because of her husband's military service.

According to the Minnesota National Guard, Brandon Horst, 25, worked full time at the 133rd Airlift Wing for nearly the past two years, was a member of the wing for six years and had been deployed to Southwest Asia.

'I want him dead'

On Aug. 4, Heather Horst arrived at the apartment Allen shared with his fiancée and two roommates.

"She said, 'I want him dead,' " Allen said. "… she finally said, 'He killed the baby.' I was enraged. I saw red."

Allen said two of his friends agreed to help but backed out. They apparently did not report the plot to police, nor did Allen's then-fiancée or a roommate, both of whom were present for some of the planning.

Allen said he told Heather Horst he had second thoughts, but she urged him to proceed.

Allen testified that he waited in the couple's basement for Horst to fall asleep and shot him between 12:17 a.m. and 12:27 a.m.

Heather Horst is charged with two counts of first-degree murder and two counts of second-degree murder in her husband's death. She is free on bail.

Chao Xiong • 612-270-4708

Twitter: @ChaoStrib