Stannis has been dumbed down for the TV viewers to the point where he's unrecognisable. A similar thing has happened to Tyrion in that he's way too nice in the show. I can't actually imagine him killing Shae in cold blood.
TBH I can't see how they can go with that arc- I can't see the TV Shae banging Tywin, or Tyrion killing her for it.
On the upside, maybe we'll get lucky and they decide to keep Tywin in the show for longer, cos Charles Fucking Dance (yeah, that's his full name) is a god among men