I was expecting a darker ending, or a darker secret to Katie...

I think that would have made a better story for Katie/Erin to have more of a past. At the start of the movie, we are shown that Katie is in some sort of trouble. But as the movie progresses, its clear she was in an abusive relationship, and she did nothing criminal just acting in self defense.

Nicholas Sparks should have made Katie's character have more of a past.

I was disappointed when they revealed she was just running from an alive abusive cop husband. It's a ripoff of the "sleeping With the Enemy" plot without an accurate portrayal of an abused woman. (For an abused woman, Erin/Katie got over her fear of men fast enough to fall into a relationship with Alex.)

Had they gone down the line of her being an actual murderer (in self defense) or witness to a murder, it would have been more original. I can see a witness to a murder who'd never been around violence or crime before being initially afraid but then falling back into a cozy, trusting nature to begin a new romance once she felt sufficiently safe in her new anonymous life.

Nicholas Sparks did a good job handling a romance and the ghostly twist, but his abusive wife plot was half-baked.