In Entertainment Weekly's interview, 'Game of Thrones' author George R.R. Martin: Why he wrote The Red Wedding -- EXCLUSIVE GRRM is quoted as saying:
I’ve said in many interviews that I like my fiction to be unpredictable. I like there to be considerable suspense. I killed Ned in the first book and it shocked a lot of people. I killed Ned because everybody thinks he’s the hero and that, sure, he’s going to get into trouble, but then he’ll somehow get out of it. The next predictable thing is to think his eldest son is going to rise up and avenge his father. And everybody is going to expect that. So immediately [killing Robb] became the next thing I had to do.
Indeed, in ASOIAF the death of key characters (on all 'sides') makes the work unpredictable to a reader lulled into a certain sense of security about the life and death of key characters in many other works (especially those that center on a single protagonist- e.g. Harry Potter). [I'm refraining from giving concrete examples to avoid spoilers.]
Is it possible that Valar Morghulis (all men must die) is a sort of meta-theme of ASOIAF and the nature of the world that GRRM has created as an author?