As far as what some kid posts on a website, I place no real credibility in it.
About those "warning signals" that a kid is going to go astray, what about Kara? She was a rebellious 14 year old, who grew up way too fast. Sending nude pics of herself to the boyfriend, having sex with him, disobeying her parents by staying out ALL night..my god..her poor parents had their hands full. She was out of control. No telling WHAT she and David were cooking up.
The police seem to be downplaying that possibility for now, but the evidence isn't fully developed yet. The computers are going to be an interesting source of information, and I'm sure the P.D. boffins are already taking them apart.
Of course, there are all sorts of intermediate possibilities - that she knew he was going to "take her away" but not that he was going to execute her parents as part of the plan.
And she does sound like an out of control kid . . . I do hope that she was not in on the murder plans, that would be WAY out of control.
I will note that an old doctor friend of mine used to say that crazy people find other crazy people to keep them company. Her poor parents, indeed! (I also feel sorry for HIS parents. It's mighty hard to believe that your son is a monster.)