A Cherokee County man who beat his sister to death outside a Woodstock home she shared with their mother pleaded guilty, but mentally ill, and was sentenced to life in prison.

Joseph Twilley, 57, pleaded guilty to malice murder, felony murder, aggravated assault and theft of a motor vehicle, the Cherokee County District Attorney’s Office announced Thursday.

Twilley killed his sister, Karen Twilley, in November 2016 outside her Pine Lake Circle home before stealing his mother’s car and driving west, Cherokee County deputies said previously.

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Prosecutors said Twilley killed the 58-year-old by striking her in the head repeatedly with a piece of wood. An autopsy also revealed evidence of strangulation, which may have contributed to the woman’s death, authorities said.

Cherokee County sheriff's deputies investigate after Karen Twilley was discovered dead outside her Woodstock home in November 2016. Her brother, Joseph Twilley, pleaded guilty, but mentally ill, to the murder.

Credit: JOHN SPINK / JSPINK@AJC.COM

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Credit: JOHN SPINK / JSPINK@AJC.COM

According to prosecutors, Joseph Twilley was previously diagnosed with schizoaffective disorder and Karen Twilley served as his legal guardian due to his mental illness.

He admitted killing his sister after his arrest, telling police the woman fell face down on the concrete outside her home after he attacked her.

“I listened to her breathe for about five minutes,” Twilley told authorities.

When she stopped breathing, Twilley covered her body with a blanket, took money from her purse, grabbed the keys to their mother’s Ford hatchback and drove away.

Investigators said he drove as far as Mississippi, changed his clothes, replaced the car’s tag with a stolen Tennessee license plate and ended up back at his Woodstock apartment.

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Twilley admitted that he hadn’t been taking his medication at the time of the murder, but a Georgia Behavioral Health and Developmental Disabilities doctor determined he understood right from wrong at the time of the slaying and found him competent to stand trial.

Twilley told the judge he went to his sister’s home to confront her about money he believed she was stealing from him, prosecutors said. He said the money was “the straw that broke the camel’s back,” and that he was still angry that he lost custody of his child after Karen Twilley reported him to child protective services nearly two decades earlier.

“She’s taken everything from me,” he told police.

Investigators determined that Twilley went to the home that morning with the intention of killing his sister, wearing gloves during the murder and even unscrewing a light outside the home.

Karen Twilley had previously told friends and family members that she feared her brother might someday hurt her, prosecutors said.

“This case destroyed a family. Twilley’s mother lost both her children after years upon years of attempting to address her son’s mental health issues,” Cherokee County District Attorney Shannon Wallace said. “Mr. Twilley was found responsible for his crimes under the law because he still knew right from wrong when he killed his sister; however, it’s a prime example of the limitations of the systems we currently have in place. The sad truth is Mr. Twilley is a tremendous danger to society. The life sentence he received is appropriate so that his mental health will always be monitored.”

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Dooley was head football coach of the Bulldogs from 1963 through 1989 and director of athletics from 1979 to 2004.