Westmoreland County District Attorney John Peck told a judge that Melvin Knight, 23, has asked to testify against Ricky Smyrnes, 26.
Peck is seeking the death penalty against Smyrnes in the February 2010 killing of Jennifer Daugherty, 30, in a dingy Greensburg apartment.
Other defendants have previously testified that Smyrnes bullied and controlled Knight who, prosecutors said, stabbed Daugherty then helped Smyrnes choke her with a string of Christmas lights.
Knight didn't testify at his own two-week sentencing trial in August. He pleaded guilty to first-degree murder, but let a jury decide whether he'd receive the death penalty or a life sentence.