Stick to cookie cutter dumb shit like Star Trek Into Darkness if you want shallow clean cut battles of Good vs Evil where the ending of the story is obvious 5 minutes in. George RR Martin strives for a more realistic, unpredicable approach, without the line between good and evil being drawn. It's what makes the series so refreshing. My favourite characters are Melisandre, Jaime, Stannis and Roose. All of which have done atrocious things. It's the charm of the series.
Robb and Catelyn were nobody's favourites so I doubt anybody will quit watching. If they'd killed Tyrion, Jaime and Arya in the same scene then maybe
Game of thrones IS cookie cutter shit with clean cut battles of Good vs. Evil.
It's just that GRR Martin likes to let the Bad guys win - for shock value. And the good guys will be made naive/etc for that to happen.
That's not more realistic than Good winning. Also, it's hardly original.