Why Mystic River Ended The Way It Did (Spoilers)

Mystic River's ending is actually pretty realistic, when you look at the neighborhood and its culture. This is South Boston, and the culture of that is the driving force that, ultimately, makes the ending of the three main character's stories inevitable.

Jimmy Markum: Men like Jimmy are considered to be the greatest people in the world in neighborhoods like his. It's the "he's a sonuvabitch, but OUR sonuvabitch attitude" that is rampant in many old and generally crime-ridden areas. No one is going to rat Jimmy out, even if they hate his guts. And it's not just the threat of being killed by him. It's the code of silence that's been hammered into everyone for years. There were at least a dozen witnesses to Jimmy murdering Dave after beating a false confession out of him. And the sad part is, Southie has infected them so much that they simply don't care who gets killed. Jimmy is the alpha male; therefore, only his family matters. Anyone who gets murdered along the way to finding justice for Katie is just collateral. Long live the king.

Sean: Sean is a good cop, but his hands are tied by the Southie code of silence. Unlike his counterparts, he has to maintain good relations with his old neighborhood to get anything done. If even one Southie boy gets put into jail with his involvement, and without the blessing of guys like Jimmy, not a damn person will ever talk to Sean again. No witnesses, no crime. Jimmy knows this, and enjoys rubbing it in Sean's face. In this world, cops are the beta male, even though the order is reversed outside of Boston. But during the parade, when Sean pretend-shoots Jimmy, it becomes clear that there are cracks forming in the old order of things. If you watch closely, the king suddenly goes from smiling at Sean, to having an angry sneer. The end may be coming sooner than he thinks.

Dave: Despite all the posters on here who believe he's a pedophile in secret, rewatch the movie and listen to his dialogue about vampires. Rather than suggesting that he became a pedophile, it suggests that he was one of those underage prostitutes for a while. His is a truly tragic story, and as usual, it all circles back to his neighborhood. When the boys stand outside his window, watch how his mother jerks him back and draws the shades. Watch how harshly people treat him years later, and transfer that to Celeste after his murder. In the world of Southie, the victim is always to blame. Everyone thinks that Dave deserved to be raped; when Sean asks Jimmy where Dave was, Jimmy tells him the last time he saw Dave was twenty-five years before in a car going up the road. To him, Dave died the moment he became weak and "allowed" himself to be violated.
The sad fact is, everyone else believes it as well. If Dave had tried to tell anyone about who had raped him, he would have been called a liar. A priest and a man posing as a cop?! How dare that little sh-t tell such lies! His own wife and son treat him with contempt; the only ones in the entire film to treat Dave with any decency are Sean and his partner. Both are outsiders due to being on the opposite side of Southie's values, and are well aware of how child molesters and rapists work. But they had to send him home eventually, and the perverted justice of the good ol' boys of the neighborhood claimed another victim.

Mystic River is a tragedy, because of the inevitability of the outcome. An innocent man is murdered, but no one will care. In South Boston, the law is what men like Jimmy Markum say it is. The victim is always evil, the violated are always liars. The most unforgivable crime in Southie is not to be evil, but to be weak.