Snohomish County jurors were told during the trial that the convicted rapist was already serving a sentence of life without parole in 2011 when he strangled Jayme Biendl in the prison chapel.
Now the jury will have to decide whether there are mitigating factors to spare the 54-year-old's life.
The only possible sentences for aggravated murder in Washington are life in prison without parole or death.
The defense didn't dispute that he strangled the corrections officer with an amplifier cord in the prison chapel of the Washington state Reformatory at Monroe.
His lawyers had suggested he didn't plan to kill Biendl, and said he blacked out in the final minutes of her life.
The jury saw Scherf's video confession and heard forensic testimony about how Biendl was strangled. 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.
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.