My understanding of the ending..thoughts?
I havent read all the posts, so sorry if this theory is already out there, but here goes..
So what we can be pretty sure of is that Sean's father covered up for Larry Childs when Sean as a boy identified him from the mugshots.
So then cutting to the end with a calm Sean receiving the red stamp of a non guilty dad, I had to think about it for awhile and then... I realised that Larry Childs was now dead because he didn't split on his mate.
Therefore the fact that Sean killed his dad was right (to him) because he was punishing him for protecting Larry for killing his Mother, therefore you can then deduce that he realises Larry Childs wont break the code of silence either and subsequently kills him for protecting his Dad. And the reason they cant break the code of silence is because each will bring the other one down with him.
We know that Larry has done a deal with the DA with his recent conviction so he's quite happy to bring the house of cards down if he gets a good deal.
Sean is depicted throughout the movie watching other people in conflict and we get glimpses of what 'he sees' as if he is trying to see the 'ones who should be protected' not in the middle of conflict, or perhaps an exaggeration of the conflict..I may be reaching here..but the point is protection and trust.
The co dependent relationship between Dad and Larry, e.g if one splits he'll bring the other one down with him is what this film is about..protection and trust. Trust and protection between 2 killers that is dishonest, trust between the retired Chief and Teresa (whom threw the gun/evidence away for her kid and her own protection) and Sean that is also dishonest..so in theory are they not all the same??
Sean is eerie at the end, perfectly calm and measured as is Teresa I wonder if they realise the line they crossed was to the side of what had started this terrible tragedy.