Why should hypocrisy, infidelity and murder ruin a perfectly good relationship?

Tom and Perri are the happy ideal New York couple until Tom's beloved Golden Retriever is killed by a hit-and-run driver. Tom blames Perri for the accident and their relationship becomes strained as Tom's fixation with finding and punishing the culprit consumes his life. Events spin out of control as death, sex and cynicism take their toll on the pair's seemingly idyllic life.