Along with ordering a new punishment proceeding for Michael D. Taylor, the court on Tuesday also upheld the death sentence for Scott A. McLaughlin for fatally stabbing an ex-girlfriend in 2003.

Taylor entered prison in 1998 on a life sentence for the 1995 rape, beating and drowning of a classmate, Christine Smetzer, in a bathroom stall at McCluer North High School in St. Louis County. Both Taylor and Smetzer were 15 years old at the time of the killing.

In 1999, Taylor was accused of strangling Shackrein Thomas, a fellow inmate at Potosi Correctional Center. The two had been cellmates for nine days when Thomas was killed.

Taylor was convicted in 2003 of first-degree murder in Thomas’ strangulation, and the Supreme Court on Tuesday upheld the conviction.

But the court ordered a new punishment proceeding, finding in part that prosecutors improperly withheld evidence, including letters from a prison informant, that showed a pattern of deceit.

The Supreme Court also faulted Taylor’s defense lawyers at the 2003 trial for failing to delve deeply enough during the penalty phase into his history of mental health problems.

Taylor had admitted to an investigator that he strangled his cellmate because “father” from the “dark side” instructed him to “send” Thomas to him. The defense argued during the guilt phase of Taylor’s trial for an acquittal by reason of insanity.

But the defense failed during the penalty phase to present all of the evidence available to it of Taylor’s mental problems and abusive childhood, the Supreme Court said. Such evidence included a threat at age 10 to commit suicide at school by jumping off a window ledge, beatings by his father and an attempt to have his “demons” exorcised from him.

Taylor has been diagnosed with paranoid schizophrenia and anti-social personality disorder.

Attorney general’s office spokesman John Fougere said Tuesday that the office will discuss with local prosecutors whether to again seek the death penalty for Taylor.

“The state is ready to argue in the penalty phase that the original maximum penalty should be assessed,” Fougere said.