See: Danielle VanDam Case.
The idea that a sick, twisted motive is not possible, in conjunction with a very protectective community is a recipee for the case never being solved.
See: Jonbenet Ramsey.
We have to consider the darkest sicknesses to get at the truth.
Right now the parents would probably be delighted if all that happen was she ran off with a scumbag boyfriend. All other options seem worse.