In Westeros, George R. R. Martin has created a dog eat dog world often dominated by war and conflict. This often results in good people doing bad things, leaving virtually none with a clear conscience. Even Ned Stark, the most honourable man in the country, has lied to those he loves and taken lives.
While almost no-one in the song of Ice and Fire saga is purely good or purely evil, of all the characters, the Lannisters sit closer to evil than most. Those who play the biggest parts in this story, Tywin and his children, Cersei, Jaime, and Tyrion, are all spread across this spectrum of good and evil.
Tywin and Cersei are the most similar in that they put their family above absolutely anything else, no matter the cost, Jaime is deep down a good man who embraces his bad reputation, and Tyrion has always tried to do the right thing in spite of his reputation. Anyone can do horrible things in order to survive war in Westeros, but this is not the same as some of the down right heinous acts of this great house.
