A man who stabbed a Walmart employee to death in 2011 pleaded guilty Monday to voluntary manslaughter and will spend the next 25 years in prison, but he refused to explain to the victim’s father why he did it.

Darius Green, 19, said he stabbed David “D.J.” Moultrie, 20, in front of Walmart in Mount Pleasant on Sept. 28, 2011.

He was charged with murder, but prosecutors agreed to have Green plea to a reduced charge of voluntary manslaughter. Circuit Judge Stephanie McDonald accepted the negotiated plea.

The courtroom was packed with the victim’s friends and family. The victim’s father, Stanley Moultrie, told the court he lost his best friend that night.

“I want to know why he killed my son,” he said, choking back tears. “I can’t sleep because of this.”

When McDonald asked Green why he did it, he repeated, “I ain’t got nothing to say about it.”

Ted Smith, one of Green’s public defenders, then explained that the incident was the end of an “ongoing beef about a girl.”

Smith said Green has suffered from “a lot of turmoil and tragedy for someone so young,” including witnessing the death of his sick mother as a young teen. His father lives in Florida and is not very involved in his life.

Green, who has an eighth-grade education, was expelled from several schools and has displayed a lot of anger and threatening behavior, lawyers said.

“He has a mental illness and a lower I.Q. that serve as a defense mechanism for him,” public defender Megan Ehrlich said. He had some problems as a juvenile but has no prior arrest history in South Carolina.

Green, who appeared in court in a black- and gray-striped jumpsuit and handcuffs, has been in jail since his arrest.