It all smacks to me of Martin being a sadist/attention whore
I don't think it's sadistic killing fictional characters that don't exist
, but maybe that's just me. Also, if you're an attention whore then being a writer is one of the dumbest career paths possible.
Robb wasn't even a POV character in the book series and Catelyn served little purpose. Both got terrific deaths and their absence isn't missed by me in the way I'd hugely miss say, Bran as a POV character.