In the book, weren't the Sheriffs musings about the nature of evil spaced throughout, and not loaded so heavy towards the final section?
I haven't read the book, so I can't compare, but his musings
are spread throughout the film, right from his opening narration. They're intercut with the Moss/Chigurh story though, such that the experience of them doesn't telegraph their thematic significance, but rather they feel like a necessary pause in the action, a chance for you to catch your breath before diving back into the suspense.
It's not so much that the film shifts abruptly in tone or genre (like
From Dusk Til Dawn) but that we're made to see that the movie we've been watching is not the movie we thought we were watching. In fact, it's a movie about the kind of movie we've been watching.
Bell is
always telling the story. We just become so involved in it that we forget that for awhile. The ending makes the film something much larger than a suspense thriller. There's nothing wrong with being simply a genre film, of course, but the Coens have more ambition than that and that's what makes the film great.