I am not real sure how the California system works, but what I recall and read from the Westerfield/VanDam case when the jury found him guilty of murder they also later had the decision of life or death. They said death, but the Judge could have changed it to life, which he didn't.
I don't see how this Judge who will be presiding over the Peterson case could go outside of the guidlines. If Scott is found guilty of 1st degree murder the choice as far as I understand it, is life or death. If the jury says death he could change it to life.