No I don't think I would have liked a narration. He would have been to creepy. I can just see a scene where he is creepying up behind someone about to kill them or something.
Also, I said this before that David was the deal with the devil for the audience. The readers probably all fantasized about being an animorph and joining up. He was a new animorph and in a few ways and audience surrogate. He even had some of the marry sue thing. But instead of the animorphs liking him, they all hated him or mistrusted him. In the end instead of saving the world like the readers would like to, he hurts everyone. Furthermore the readers can assume they probably would have done the same in his case.