Scary as it is to accept, I think he was calm because he did exactly what he intended to do and achieved the result he had hoped for. And that is frightening as hell.
I don't see mental illness at all. I see control. Abject control.
Not an act of desperation, not a crime of passion, not some misguided delusion -- his method grotesque, his motive simple -- he wanted to hurt his wife spectacularly.
He might have every inbred personality disorder there is, but he is and was IMO fully sane. Just altogether disordered. Living out some corrupt moral code of his own making. Evil, vindictive, void of compassion. Void of humanity. Void of all decency. Barking orders at a dog when he'd just created utter mayhem, the likes of which a dog could never understand. Gunning down his own babies, reign of terror, in broad daylight. There's more humanity in that traumatized canine than in this sorry shape of s father.
That is one sick, jaded, nasty cruel streak, and if it buys him 3 LWOPs, it won't come close to justice enough.
Jmo