I tend to agree with you that they probably didn't have anything directly to do with her killing, but they "seem" to know more about it than they are letting on. A couple posters here have seen some interviews with the parents and thought something just wasn't right.
Purely speculative of course, but being that they were apparently into some odd sex stuff I wonder if they made some deals while under the influence that may have led to this.
Yeah, I've heard some speculation about that too. I think any of their background will eventually come out as this unfolds. . .