A Missouri man was set to be executed Tuesday for killing his girlfriend and her young daughter during a violent argument.

Defense attorney Jennifer Herndon said Richard Strong "just snapped" during a heated exchange between two people with mental health issues. He used a butcher's knife to kill Eva Washington, then turned the weapon on Washington's 2-year-old daughter from a previous relationship, Zandrea Thomas.

Strong, 47, was scheduled to die by lethal injection at 6 p.m. Tuesday. He would be the fourth man executed in Missouri this year and the 16th since November 2013. Only Texas has performed more executions over that span.

"Just shoot me, just shoot me. I killed them."

- Richard Strong told police during his arrest, court papers say

Police in the St. Louis suburb of St. Ann received a 911 call on Oct. 23, 2000, and heard a scream. They went to Washington's apartment. Strong met them outside and told them at first that Washington was sleeping, and then later that she had gone to work. 

Officers saw blood stains on his hand, and Strong tried to run. When they caught him, he admitted to the crime.

"Just shoot me, just shoot me," he said, according to court records. "I killed them."

Inside, police found the bodies. They also found a pool of blood on a bed, the knife next to it, and the couple's 3-month-old daughter, Alyshia Strong, who was unhurt. Herndon said both Strong and Washington suffered from mental illness and frequently argued.

"He just snapped," Herndon said. "It was just sort of a powder keg waiting to explode. It wasn't a healthy relationship."

Alyshia Strong was taken in by Strong's mother and, despite the crime, grew close to her father, frequently visiting him in prison. A clemency petition to Gov. Jay Nixon relied heavily on Alyshia Strong's words describing the importance of her father in her life.

"l know some people probably wonder how I can have a relationship with my father given that he killed my mother, but we are very close. ... I am thankful I have him in my life," Alyshia, now 14, wrote in the clemency petition.

"I've never been angry with my dad, and I've learned to forgive," she added in an interview.

A spokesman for Nixon said the governor was still weighing the clemency request. Meanwhile, the U.S. Supreme Court was considering two petitions seeking to halt the execution, one procedural, and one citing the fact that Richard Strong suffers from severe depression.

Alyshia said in the clemency petition that she thinks "it is wrong for me to have another loss. I understand that my father needs to face consequences and to pay for what he did, but I do not think it is right for me to lose my father as part of the punishment."