Escape

The story is edge-of-your-seat gripping, quite easy to slip into and believable. About halfway, improbabilities start creeping in. Perhaps they're worth it, because they cause you to shift your focus from the actions to the minds driving them.

The psychopathy becomes the story. The (preachy!) longing to be free becomes an afterthought. We suspect unrequited love from one protagonist, but the relationship is never fully defined. The lovelorn one makes it clear he wishes to keep his object close, though. The rejection is profound, and the pet that turned ugly must be destroyed.

By the story's end, improbable becomes ludicrous. Still, if you hang on to the psych angle - divorce it thoroughly from any chance at reality, it's palatable.