First off, this movie portrays Karla as a victim, an abused wife just trying desperately to keep her husband happy. That's pretty inconsistent with the facts, and the tapes that were found showing her to be a willing and active participant to the kidnapping, sexual abuse, and torture. She was a sociopath that worked the system for lenience, and that's pretty much universally agreed upon.
Storywise, there's no chemistry between Paul and Karla, and instead of the happy, beautiful couple that the real life Ken and Barbie Killers put up, we get a mopey wife, and an irrationally abusive husband. There's no introspection here, no psychology, no attention to motivation.
The result is a poorly developed serial rapist and killer paired with an unquestioning victim, and the audience is left wondering why. In one scene, a victim asks Karla why she stays, and she says, "You don't understand.". Well, nobody understands, and the movie misses an opportunity to explain why.