All the evidence points to his planning this, even calling his co-worker the night before to make sure he'd have that field and oil tanks to himself. The late plane and NUA's intervention made his plan fall apart. But he planned to murder his wife and his children. In his depraved, sick, cold mind, they all had to go. He is a Family Annihilator if you want to categorize, not "just" a wife killer (and I don't really mean "just," it is all horrific). His intention, his plan, was to be rid of them all, and to make it seem that Shannan took off with the kids, simply "vanished. Nothing here." His planned speech to the TV cams didn't make much sense. He claimed to come back to "nothing" as if all her stuff and the kids had cleared out, but he hadn't had time to clear it all out yet. I think it would somehow be easier to comprehend if it were a moment of rage, but that is simply not what happened, according to the DA, his confession itself, and all evidence.