The jury was tasked with determining whether Scherf, 54, planned the killing of corrections officer Jayme Biendl, 34, or if it was just a violent attack.
Prosecutors say they plan to pursue this as a capital murder case.
A jury takes up the sentencing phase on Monday.
Scherf and his lawyers did not dispute that he strangled a corrections officer to death with an amplifier cord in the prison chapel of the Washington state Reformatory at Monroe.
Scherf is a convicted rapist who was serving a life sentence in January 2011 when he killed Biendl He initially cooperated with investigators and confessed but stopped because he says he was denied jail privileges that had been promised. His trial began May 1.
After closing arguments Thursday, the jury had the option of convicting Scherf of second-degree murder, which would remove execution as a punishment, The Daily Herald reported.
Scherf's lawyers suggested he didn't plan to kill Biendl, and they say he blacked out in the final minutes of her life.
The jury saw Scherf's video confession this week and heard forensic testimony about how Biendl was strangled with an amplifier cord. Scherf said he first planned to ambush and beat up Biendl over something she said to him. He refused to say what set him off.
On Wednesday, Snohomish County Medical Examiner Norman Thiersch testified it would have taken four to five minutes of constant pressure to strangle Biendl. It took significant force to fracture the cartilage in her neck while she struggled to survive, he said.
Scherf's DNA was found on Biendl's fingernails. His blood was found on her coat and the amplifier cord, forensic scientists from the state crime lab testified.
Scherf's hands were bright red with an obvious mark across a palm, testified a Monroe police detective who saw him a few hours after the killing.
Scherf said he blacked out while pulling on the cord. When he came to, he was sitting in a chair in the back of the church sanctuary. He told detectives he had no memories of Biendl dying.