With a double murder charge, the kidnapping (and likely statutory rape) charges are small potatoes. Get him on the big charges, put him away forever.
I heard on the local news here that in PA, to get a death penalty, a murder charge must be accompanied by another felony charge which kidnapping is. I'm not a lawyer and this didn't make sense to me, but that seems to have been the reason that whether or not the kidnapping charge stuck was so important.