He will only take the stand if he :
a) if he is guilty and admits its or
b) not guilty and so states from the witness stand to let the jury know they made a big mistake and therefore spare him his life.
If he is guilty and does not admit it the prosection will rip him a new one on the stand.
I think what you are referring to is the "penalty phase" of the trial. Courtesy of the US Supreme Court rewriting the Constitution, the jury -- rather than the judge -- now must decide what penalty to apply for Westerfield's offenses as found in the "guilt phase" of the trial.
Rape and murder with aggravated circumstances warrant the death penalty under California law. However, all it takes is a single juror who is too squeamish to do what the law says, and the death penalty is out and life without parole is the result. Westerfield can take the stand in the penalty phase, but he is unlikely to do so because the prosecution could then question him about the blood, the hair, the pornography, and the motor home trip.
Look for Westerfield to remain silent and for the defense attorneys to argue, in effect, "He didn't REALLY do it, even though you found that he did do it." Their goal will be to find just one squish juror to vote against the death penalty.
Congressman Billybob