Insanity
One of the main focal points of the plot is that Maureen is schizophrenic and prone to violent, insane episodes. This doesn't seem to be the case, though, and the reader wouldn't have guessed that about Maureen, but unfortunately, in the fullness of time, the reader is left with a pretty grim opinion of the whole affair. It's tragic in a way, because Maureen might have become healthy if she would face her problems head on. Perhaps her mental illness prevents this, but in the end, who knows? Maureen is a mystery to herself, and to the reader.
Psychological horror
The prospect of not being able to trust in one's perception of reality is really horrifying. So horrifying in fact that instead of being able to face that basic fact, Maureen is willing to torture her own mother into a false confession. Maureen is so intent on ignoring her mental health issues that she is even willing to kill.
The complexity of a relationship
In many ways, Pato is the victim of the relationship, but he doesn't even know it, because Maureen is so talented at behaving in a way that prevents him from becoming suspicious. But this means that so long as someone can lie convincingly (to themselves as well perhaps), then that person could be anyone - perhaps a psychopathic murderer, as the plot illustrates.
