Percentage-wise I would go with the random killer.
The parents BEHAVIOUR shows more of a GUILTY because the didn't pay attention to why they hadn't seen Danielle for hours, and then, hoped she would show up, be OK, then started realizing she had been gone too long, and their whole family, and life were on the chopping block if they ever let out the truth.
Damon may have been the first to know and kept it from Brenda until Sat Morning.