Man, I saw this when it first came out, and don't remember.

I just watched it for Haley Bennett.

@TCap111 said:

I need the end explained. Did the young husband kill his wife? What was the motive for the older husband to kill the younger one?

I wondered the same thing. I think that his smile at the end was the director hitting us over the head with the "hint" that he did kill her and got away with it and got the over guy killed (to uncomplicate things maybe?). And I don't think it matters why the other guy killed his wife. I think the big hint there was her harping on him in that one scene before he killed her. Also the synopsis on the guide (Comcast) said that he was imagining killing her. I got the impression from it that it was all just a fantasy. But after seeing it, other than the smile from him, it seems like she did kill herself. And I was confused whether he really had the affair or not. I guess he did. We just didn't know it was happening at first. I don't know - I liked it because I love Patrick Wilson and film noirs, so the combo was great.