The script of Mystic River piles on the elements leading to Dave's death but they don't inform each other. The psychological disorder caused by his molestation makes it believable that Dave couldn't credibly defend himself against the false suspicion of murder, and since the murder he did commit is one he'd rather not talk about he does look guilty, but there's no thematic connection between child molestation and Katie's death. The makers want us to feel that the crime committed against Dave makes it inevitable that he be falsely accused and punished, that having been molested marks him the way his weakness as a boy marked him for the pedophiles in the first place. If that's the case, however, then his murder of the child molester is unnecessary and in fact dilutes this meaning because coming in bloody the night of Katie's death would cast suspicion on Dave whether or not he'd been raped as a boy.

From another perspective, Dave murders the pedophile that night because he'd been raped as a boy, but that's simply a plot connection, a way to make it plausible (and supposedly excusable) that he was out killing somebody that fateful night. (It leaves open the possibility of thinking, "Damn, if only he'd murdered the pedophile a week later!") That is, the various baleful actions in the movie are a pretty random assortment jury-rigged for plot purposes. (And not that sensibly: wouldn't it all be tighter if Jimmy's son had been killed?) Though Sean and Jimmy at one point speak elegiacally about Dave's abduction, saying it's as if all three of them had got in the car, the abduction has nothing essential to do with either Katie's or Dave's murder. It might as well have been aliens who abducted Dave; it could have been anything that set him up to be misunderstood.

There should also be some significance to the actual killers' motivation. Instead, Katie turns out to have been killed accidentally by her boyfriend's mute brother and a friend, waving a gun left in their apartment by the brothers' criminal father before he disappeared long ago. That father was the man who turned state's evidence and sent Jimmy to prison, for crimes committed by the Savages. When Jimmy got out of jail he murdered him (in the same backwater where he later kills Dave) and he had told Katie that she could never go with anyone from that family. But the boys didn't know about their father's relationship to Jimmy, and Katie's death was an accident.

So the family rivalry and retribution plot is woven into, and yet entirely incidental to, the contemporary action. (This is a movie with a veritable school of red herrings swimming through it.) Thus, at the heart of this supposed tragedy is a meaningless coincidence. In this interview with The Boston Phoenix, Helgeland compares the script to the story of Oedipus, but Oedipus's killing of his father wasn't a coincidence; it was fated so that even though it was foretold to him he couldn't avoid it by no matter how much effort. In Mystic River the fact that the killing was an accident means there's no sense in which Jimmy tragically set his daughter's death in motion by his own behavior earlier. She, too, might as well have been harmed by aliens.