In the novel, the boy speaks when he comes back. In fact, he uses the F-bomb. In the movie, he talks like a little kid ("Now I want to play with you!"), which is much creepier.
As for the book, well (for me) it was pretty clear the wife would kill the husband. For me, it was Stephen King using "it" as the wife's pronoun the led me to that conclusion. "Darling," it said. It. Not a nice "she" (if she was back and 'normal'). No, he used "it" so it's not human--it's now undead and will kill him. That was my feeling as I finished the book (more than 20 years ago, though).
I had also read an interview with King, where he said that he wrote the book years ago, and then put it in a drawer because it was "too scary." At the time, I certainly enjoyed the book as a 'return to form' for King, who has been sliding into lower standards (Firestarter, Christine, etc.)