Some reviewers seem to miss the point slightly...

Rating: 8/10 (31 August 2005)

This is a very multi-layered film that seems to revel in confronting the viewer with moral ambiguity. The upshot of this is that it becomes thought-provoking and causes self-analysis in the viewer, the downside is that it's not the kind of movie you watch just to kill a couple of hours.
Firstly I'm surprised that people are demanding a change in the ending... the film ends the way it does because the book ends the way it does. Mystic River's novel origins can also explain why certain aspects of the story, such as Sean (Kevin Bacon)'s relationship with his wife, aren't as fleshed out as they might be.
There are also complaints that the way Dave (Tim Robbins) is shot because it 'makes him look guilty'. Surely that's the whole core of what allows the audience to sympathise with Celeste (Marcia Gay Harden)? Her already damaged husband comes home covered in blood, glosses over how it got there and steadily becomes more and more erratic over the course of the following days. Importantly, the audience is never given a clue that he may be innocent of Katie's murder, and neither is Celeste. This makes her confiding in someone understandable, even if Jimmy (Sean Penn) probably wasn't the person to confide in.
The story ends where it does to allow the reader to make up their own mind. This was never going to have the pat Hollywood ending with Sean racing to Dave's rescue, probably accompanied by a five-minute car-chase. It is this that makes the story believable, if not entirely likely. It is never explicitly stated that Sean will let Jimmy get away with Dave's murder - in fact the 'gun finger' shot is probably one of the most ambiguous set-ups in the movie.
Anyhoo, my interpretation, with added spoilers: Without a body, the only evidence against Jimmy was circumstantial. It was already established that local law enforcement doesn't have cutting-edge CSI techniques available to them, as they couldn't do a DNA test on the blood in Dave's car. I suspect it would also weigh on Sean's mind that putting Jimmy away will deprive Katie's two younger sisters of a father as well as a sister.
Jimmy puts a brave face on in the final scene - but is he really as sanguine as he appears? He demonstrated on the porch that he's not just a heartless thug.
As has been pointed out before - Ray Jr. killed Katie because he didn't want her to take his brother - the only person that actually spent any time with him, remember - away... a boy who can't speak ends up speaking out in a terrifying way.
Though it's never vocalised, I suspect that Sean told Jimmy that the shooting was an accident to prevent him from taking it out on older brother Brendan - after all, had they not been secretly dating, they wouldn't have been planning to run away and Katie would still be alive - It wouldn't take much for Jimmy to reach that conclusion.
All in all a good film to watch, but those in search of a happy ending would be advised to watch elsewhere - this doesn't detract from the film's quality as a cohesive whole though.