I don't quite agree with that. It's a bit less clear when you start dissecting the story. The biggest thing I believe that supports the theory that he didn't kill her is the last sentence. Particularly the word stupid. If he wanted his wife to be scared and start freaking out wouldn't turning on the light be the climax? It seems to be the anticlimax though. To me this signifies that the wife's fear is tamed. Because when someone stupidly turned on the light ( I believe it to be the girl) the wife saw her. There are other areas too. Like how he calls the children mice. Showing how they can be manipulated and controlled like lab rats. Also the mention on leaves in that same sentence signifying change. Aloso, wlike you point out, the witch he says is finally dead. His daughter is a skeleton. That doesn't make sense. There are more too.
What I picture happening is that the husband was going to take his daughter away from his wife's "side" and bring her to his. Think about it. He feels alone. He's felt alone for years, and he wants to make his wife feel like that too. So why not make his wife feel like the odd one out for a change. So he concocts this "masterful" plan to get his daughter on his side but it falls through when the daughter stupidly turns on the light and chooses her mom, again. There should be a sequel to this.