What I think happened to Nick
Beautiful movie! But I don't think Nick's fate was left in any way ambiguous. The last time Nick sees Dawn, Nick screams that David *beep* her but loved him. The last think Dawn says as she's storming out is that if Nick loved David he'd leave her and David alone. I think that is what exactly happened. But he couldn't plausibly just leave. He was heartbroken and lonely and he also obviously suffered from PTSD. It makes complete sense for him to fly into a rage and tear and break things, and try to scratch his father from the snapshot we see Dawn looking at when she comes back later to give Nick money. And then, he left. Because he truly loved David and wanted him to be happy. I actually didn't get the impression that he was upset or hurt when he said David had owned him. I felt he was proud.
I actually think David is a hero. If I understood correctly (had a little hard time following all the British), he is a victim of abuse. He is not a cold-blooded murderer; he and his younger brother were abused and he actually took on the mammoth task of protecting his brother. Throughout the movie he seems to take care of Nick, he is generous with physical affection (he would, of course, since for me the incest was leaping out from the screen), he loyally refused Dawn's request to get rid of him. His violent outbreaks are rare and only happen when he is incredibly provoked. Dawn doesn't have a single bruise anywhere. Letting the dog out, lying about his mother, omitting to mention his father, and telling Nick Dawn was the infertile one -- bad stuff. He didn't escape unscathed. He's scarred, and he lashes out, and he lies, and he completely loses it. But very, very rarely.
In the final scene, I didn't feel Gary was trying to rub it in David's face. I felt it was suddenly obvious to both Gary and David and Dawn (I don't think Gary's wife caught on) how very closely the baby resembled Gary. The baby actually really did resemble him very much. If you look at Benedict's face you'll find all the pieces falling into place visibly on his face. Gary on the other hand was stuck -- I felt he was envious, then he was curious whether anyone caught on, then he became increasingly uncomfortable toward the end as though realizing that they had indeed caught on. And finally the look David levels at Dawn while he continues carrying the baby... I felt it was a combination of hurt, resentment at having been made a fool of for no reason (true, because who in real life runs to their partner and dutifully informs them they can't have kids? He didn't deserve to be cheated on and with someone like Gary no less) and perhaps the beginning of hate. I felt that his deep love for Dawn which was palpable throughout the movie began to dry up in that final scene.
Having said all that, I usually see things that seem very obvious to me only to discover no one else thinks they make sense. So if you've made it this far down this post, please know I'm not ridiculing other explanations of the movie. I was simply adding my two pennies' worth and gushing about another of Benedict's awesome movies.