I'm not sure there was anything as overt as a full-on assault, although I also think SA is the most likely culprit.
In SA we have someone who is socially immature, feels isolated from his actual peer group and whose few friendships tend to be remote (internet forums) or age-inappropriate (younger cousins, etc). In BD, he finds at last someone who to some degree shares or at least does not object to his interests in horror and role-playing and who seems to defer to him as male presence, rule-maker and so on. Suddenly he is powerful, and in charge.
I expect that, confronted with his various creepy behaviours, SA is likely to have claimed that he was "just joking," or told HD to "lighten up," hoping to invalidate her feeling threatened, uncomfortable, even endangered. Then it all comes to a head: he crosses a line and she calls him on it, perhaps more aggressively or (to his mind) humiliatingly than before. A scuffle ensues; she is injured or dies. Then he scrambles to cover up, and, incredibly, the case is investigated so clumsily that he does, for awhile anyway.
I have no idea what weird codependency or perhaps LE-sponsored strategy is at work in BD's seeming support of and continued contact with SA. But I'm sorry to say that I think HD has been dead for some time, that she will eventually be discovered, and that then charges will be laid. I do not think that SA is a deranged sociopath with a trail of bodies behind him or a criminal mastermind who has been able to outwit LE and searchers.
I think this will turn out much like the Powell case and some others: a nasty but largely unpremeditated crime followed by a cover-up that is two parts dumb luck and one part investigative failure. A nasty, depressing, unnecessary crime.
IMHO, of course.
s