His trump card to avoid the death penalty will be telling them where she is.
That’s almost a given.
He will probably get a second degree murder conviction, in exchange for her whereabouts.
He can still be put away for the rest of his life if he is convicted of all the charges consecutively if he is convicted.
It didn’t save Ted Bundy. He told the authorities he could show them more victims. He gave enough and they said thanks but your wasting Oxygen. Then executed the SOB.