The problem with Affliction is one which has bothered Cranky all his life. I don't like stories that don't have an ending. Stories that leave enough of a hole that you have lots of room for discussion and speculation. It's even worse when the writer, as is the case here, deliberately says that they have nothing but an assumption as to what happened. Almost as an afterthought we learn that an important supporting character has been killed by one of the principals.
Stories are over and the murder is just part of an epilog.
When the whole point of the piece is to show the total breakdown of the principal character and the effect on everything around him, you as a ticket buy shouldn't have to figure it all out after you've left the theater. On average, a first run movie ticket will run you Eight Bucks. Cranky didn't like this kind of storytelling in film school. He doesn't like it any better as a consumer.