For Sean to not take any police action knowing full well that Jimmy has murdered Dave is totally out of character which throws the entire movie off its hinges. That's just UNFORGIVABLE!!
The ending would have been that much much better if Jimmy had overcome his worse nature and decided to keep his word and let Dave go. On the way back he is overcome by anger at his 'weakness' and felt he had let his daughter down. Then hours later he finds out that that had been the right thing to do as the killers were someone else. He shared it with his wife ... and what she actually said in the movie and the ensuing seduction would then be redemptive and not just weird and yuckky as it is.
The ending as is, is just a badly executed cheap shot at last minute 'shock' value which totally betrayed the movie up to that point.
The ending would have been that much much better if Jimmy had overcome his worse nature and decided to keep his word and let Dave go. On the way back he is overcome by anger at his 'weakness' and felt he had let his daughter down. Then hours later he finds out that that had been the right thing to do as the killers were someone else. He shared it with his wife ... and what she actually said in the movie and the ensuing seduction would then be redemptive and not just weird and yuckky as it is.
The ending as is, is just a badly executed cheap shot at last minute 'shock' value which totally betrayed the movie up to that point.