From what I've read, the now-condemned Scott Peterson went to church from time to time, so maybe he's familiar with this verse:
"Whoever sheds the blood of man, by man shall his blood be shed; for in the image of God has God made man."
That's God commanding Noah after the flood, laying down one of the few laws given in the book of Genesis. Many people, even some Christians, have a completely twisted view of God and think that because man was created in his image we should never take another man's life; in fact, as can be seen in this verse, our Godlikeness is exactly why capital punishment is required by the principles of justice.
Here's some death row statistics from the California Department of Corrections from 2002.
Total Received (1978 to Date): 717
Sentence overturned; resentenced or released: 60
Death sentence was from another state; returned to that state: 2
Considering two inmates are living who were condemned in 1978, Scott Peterson probably has quite a lot of life left in him. The 10 inmates who were executed spent an average of 16.03 years on death row before meeting their fates.