Jaded and Faded
Detective Somerset is the veteran detective, world-weary and tired. He's the classic "one last case!" type. All he wants is to wrap up this sicko serial murder case and retire, leaving the city to go live in a farmhouse somewhere. And, of course, this last case is extra intense.
Somerset is a good detective, but perhaps he's crossed the border from cool and calm into apathetic and jaded. This is one reason he initially butts heads with Mills, the young hotshot assigned as his partner for his last week on the force. He treats Mills like a flunky, shooing him from the crime scene to interview neighbors. "My badge says detective, same as yours," Mills says. He wants to be treated fairly, continuing, "Just don't jerk me off, that's all I ask. Don't jerk me off."
But, the two slowly warm up to each other. It helps that Mills' wife, Tracy, doesn't have the same need to prove herself that her husband does. Mills and Somerset become friends after Tracy invites Somerset over for dinner. And Tracy, not having any other friends, confides in Somerset when she is pregnant and too worried to tell her husband. Maybe somewhere under his tough exterior, he has a gooey center.
Getting the Heck Outta Dodge
Somerset may be warm toward Tracy, but he remains a little standoffish toward her husband. As he says at dinner:
"Anyone who spends a significant amount of time with me finds me disagreeable."
And where he and Mills disagree most is over human nature.
Mills cares about helping people. Somerset, who has many, many years on Mills, has mostly given up. He wants to get out of the city, which he views as a total cesspool, much like humanity itself. At a bar, he describes how he views his job in one of the movie's most dramatic exchanges of dialogue:
"Picking up the pieces. We're collecting all the evidence, taking all the pictures and samples, writing everything down, noting the time things happen. […] Putting everything into neat little piles and filing it away on the off chance it will ever be needed in the courtroom. Picking up diamonds on a deserted island, saving them in case we get rescued. […] Even the most promising clues usually only lead to others. So many corpses roll away unrevenged."
Dang. That's pretty hopeless. But Mills doesn't believe it. He calls Somerset's bluff, saying, "Bulls***. […] Don't try to tell me you didn't get that rush tonight. I saw you. We're getting somewhere." And, perhaps Mills is right. Somerset agrees to stay with the police force until the case is solved instead of just retiring in the middle of it.
Somerset's shield of cynicism ends up protecting him. He has no wife or child that Doe can use to get to him, like he does to get to Mills. For all of Somerset's hatred of apathy, it is his own apathy that ends up saving him. How bitterly ironic.
But maybe—and it's a tiny maybe—Somerset has changed a bit by the end of the movie. In an odd bit of voice-over, the only one in the film, Somerset says the last lines:
"Ernest Hemingway once wrote, 'The world is a fine place and worth fighting for.' I agree with the second part."
Maybe he'll continue fighting, even though he still believes the world is a pit. Maybe he'll fight for Mills, or the memory of Tracy, or even just because fighting is ultimately better than rolling over and giving in to apathy.