This was a good and thorough resolution to this series, or season as we say in the US. Must say I'm impressed with this show and how naturally and subtly the story is told. Perhaps because it is tiring that so much television has become about conflict through reality show level bickering, it is refreshing to watch something that feels more realistic and less exhausting. There are lots of funny tinged murder shows like The Mentalist or The Closer. This has more of a relaxed tinge. It is also nice to see more details of what happens after the killer is revealed than most crime shows provide, especially with the other suspects.
As to the question of why the son is so angry, there's a scene about 30 minutes in where he says "... and realised that this is who we are now." So in part at least his anger seems to be that scandal of events will follow the family. Of course, it's also believable that someone who grew up with two parents keeping big secrets and who also have had clear anger issues, that it could have been an unhappy household growing up and lead to another person with tendencies to aggressive anger. I'd worry what that man's family would have to deal with in him.
As to the question of why the son is so angry, there's a scene about 30 minutes in where he says "... and realised that this is who we are now." So in part at least his anger seems to be that scandal of events will follow the family. Of course, it's also believable that someone who grew up with two parents keeping big secrets and who also have had clear anger issues, that it could have been an unhappy household growing up and lead to another person with tendencies to aggressive anger. I'd worry what that man's family would have to deal with in him.