Mr. Brooks, while sometimes a messy convolution of plots and ideas, has a very interesting theme.
Naturally, spoilers are ahead.
The character Earl Brooks, portrayed by Kevin Costner, has an addiction. He is addicted to killing people. A destructive habit that is clearly parallel with alcoholism. In fact, Brooks attends AA meetings and confesses to being an "addict" rather than saying "Alcoholic." The habit itself is a man named Marshall, played with glee by William Hurt as a hallucinatory manifestation of Brooks impulses and more or less his dialogue with himself.
Brooks clearly derives a great of pleasure, perhaps even beyond sexual, from killing and Marshall only encourages him and reminds him of the rules that have prevented Brooks from being caught for years.
This dynamic fascinates most during a key scene where, in a truly interesting twist, Brooks finds his daughter is suspected of a murder.
Brooks is visibly shaking after the police leave. Marshall asserts that the girl was sloppy and stupid and deserves to be caught. Perhaps jail will be good for her. Brooks doesn't agree and in fact is so stoic throughout the movie that seeing him in the state is rather wrenching. He crumbles to the floor in agony, asking "What if she has what I have?!"
There is a moment where Marshall actually hugs Brooks, consoles him and tries to help.
The whole scene is amazing because it embodies so many things. The most obvious is parental anxiety over children making their parents same mistakes.
Recalling a scene in the Sopranos where Edith asks Tony "How will we save these children?!" knowing full well that their lives are built from the brick and mortar of corruption and violence.
Brooks' anxiety is on par with that, but so much sadder. A man concerned for his daughter's well being not because it's a dangerous world or because she has made mistake, but terrified to a breaking point that his daughter has the insidious disease that scrapes at the back of his mind. He knows that what he does ruins lives and spreads misery. He knows how isolating it can be. No one truly knows who he is and he can't really tell anyone. Those who find out are doomed, because despite his existence he will still seek to preserve his well being.
How will she cope? Can she cope? Isn't murder a man's game? How will she be able to do the only thing that makes men masculine in this modern world? It's a "hard heart that kills," and the very idea of that hardness, that coldness in his own daughter literally brings Brooks to his knees.
What is truly great about this is Marshall, the monster consoling him. In this way, the coldness and the capacity for violence become Brooks' only solace and in the end the only way he can think of to save his daughter.
It's truly a terrifying concept, and not altogether alien, the idea of men using violence to save their families. In Brooks' case, it's in many ways even worse because it represents him turning to his addiction to violence to save the next generation who may also be addicted. Is she even worth saving? She'll have the same life as Brooks. It will be lonely and if she is half as careful as her old man, it will damage society. But how can a parent turn their back on a child?
There is the real horror; when a parent must choose between what is morally right, what is clearly better for the world and the life of their child. It is a terrible decision and either choice will surely cause Brooks, or any parent for that matter, to loose sleep.