I think he's guilty, too. However, I am not convinced it was planned ahead of time. I'm open to various scenarios at this point. With that said, if he did, for example, get into an argument with his wife and kill her, he then made a conscious choice to kill his children. That's premeditation.
One thing I read in my new issue of People magazine this evening struck me: "Text messages and search histories were not deleted."
Now, this guy might not be the sharpest tool in the shed, but c'mon. If he deleted SM a week prior, he wouldn't have the sense to delete his search history or texts ahead of time if he was planning on murdering his family? This points strongly to unplanned IMO.