We know this because we spend somewhere between a third and half the book's length inside the killer's head. King writes in the narrow third person, predominantly from two points of view - that is, he jumps back and forth from behind his two lead characters' eyes, reporting on what they see and feel. His technique is impeccable, although he also combines present tense narration with a fairly high reliance on flashbacks, meaning the on-rushing flow of the story is constantly being broken in a way I found jarring.
But in the main and on the whole, King is simply a masterful storyteller. He's easy to read, though not, here, as hard to put down as usual. He reportedly began this book as a short story, and it feels almost too small to be a novel. Its shape and ultimate destination are clear from quite early on, which dilutes the suspense and leaves the characters to hold our attention. Or not. Which brings us back to those sex scenes.
An ageing man, likeable and down on his luck, gets an unexpected shot at a life with a gorgeous, wealthy, much younger woman. They have excellent sex. It's pleasant, but a bit too obviously an ageing male's fantasy and it left me rolling my eyes slightly. Meanwhile, a stunted disaster of a man crawls into bed with his ageing, drunken mother and relieves his urges, thus clearing his mind sufficiently that he can go back to planning murder.
This is a different form of fantasy, the fantasy that says, "Because this is unpleasant, it explains this man."
Deep psychology is not King's strength, though his characters generally have shoulders strong enough for whatever weight he needs to place on them. Here, the story is a little thin, and the characters have to carry a little too much. They have to carry the whole book. They didn't quite carry me.
Mr Mercedes by Stephen King (Hodder & Stoughton $39.99).