[Note: This mail is a continuation of the discussion in this message.]
I do understand your point, but even among the Lannisters, Tyrion who was by far the best of them (well Jaime is getting close now) was still working for the family good even if it was not a good goal for the rest of the Seven Kingdoms or the Starks. I always thought that Tyrion was sort of the good guy who works for a bad cause.
Remember, there are many Lannisters you haven't met yet -- Lady Genna, Ser Daven, all sorts of cousins... it's a large family.