Now this is coming from someone who thoroughly enjoyed the first two books and the series too...
The killing off characters is getting old.
No, all jokes aside the characters I truly sympathise with all die, generally pretty horribly. I think it was around the Red Wedding that I just grew disillusioned with the whole thing. I gave up on the books aroun Storm of Swords and the show at a point not much later.
They are very well written, but other than Tyrion, there's really not many left I actually like. Plenty that I don't mind and many I find very interesting, but not many I actually like.
As to Joffrey, I agree with what was said regarding his upbringing being the cause of his problems, rather than his incestuous conception. What we know is that as a child he attempted to win his "father's" approval with violence (The cats. Was it cats?) and he was rebuffed for it. To Joffrey, he saw his "father" take what he wanted (The throne) with violence and so therefore decided that to emulate the man he admired, he had to do much the same. In essence, I think that when he attempted to win his father's approval with acts of cruelty, in essence, Joffrey was trying to appeal to what he saw and had heard of his father, a mighty warrior who took what he wanted by force.
As to Cersei, I have very little sympathy for her. And I only have a little more for Robert Baratheon. I would explain my reasons, but it is really late over here, so good night.