SPOILER WARNING...I figure red is the color of passion and also the color of blood, and the film uses it liberally because it equates the two. The ending only makes sense to me in the context of that subtext.
Frannie is surrounded by men who are attracted to her and who seem like they could be the murderer, but the one who really is is the one who seems least likely, by approaching with smiles, songs, and a ring. Love itself is symbolized by this story of how Frannie's parents got together; it's a story of betrayal, later re-envisioned as an act of careless murder. There's also this constant barrage of imagery (the 'Mom' funeral bouquet, the poetry, and the bride in the subway, Frannie's courtship charm bracelet, the toy that sings "I Think I Love You", etc.) that's reminiscent of our romantic ideals, and how out of place they seem when compared to reality.
Meanwhile, every character in the film is somehow a victim of their relationships--alpha male Malloy is sleeping on the couch, his partner had his gun taken away, Pauline has a court appointment, and Frannie has the whole film to wander dangerously towards eroticism and away from reality. But which main character has to die? The one who can't keep herself from calling her lover.
I love this film because I think that the idea is interesting, and from my own experiences, seems true. Passion invites everyone to flee from their own intelligence. I have female friends who liked In the Cut for the same reason, but I don't expect everyone to.
Also, Meg Ryan is fantastic in this. For years, the only film of hers I liked was Innerspace, but I'm a big fan of this film, as well as Hurlyburly and Kate & Leopold, and she's great in all of them. This film is also the first place I saw Mark Ruffalo, and he's incredible--in general, but particularly in this one. The music and cinematography in this film are also very good.