In regards to this comment on Joffrey: 'Although not a Targaryen himself, Joffrey Baratheon's incestuous bloodline is thought to have produced similarly severe defects in his sanity; he does not, in fact, have a drop of Baratheon blood in his veins. Joffrey's parents are actually twin brother and sister, Jaime and Cersei Lannister. While he doesn't hear voices or see hallucinations, Joffrey is a sadistic, megalomaniacal sociopath.'
I don't think this is a good analysis. A lot of these personality defects can be traced to his parentage, having an absent father figure, who on the rare occasions he appears just uses negative reinforcement, and more often is just in the other room shagging multiple women while drinking to excess, and indulging every lurid fantasy he comes to fancy, which would destroy the kid's perspective on what it means to 'be a man' AND what it means to 'be a king'. And then this is coupled with his mother babying him into adulthood and whispering in his ears how much he's entitled to (everything) how little regard he should have for ANYONE ELSE EVER, and the fact that she's probably the more well adjusted parent of the two.
A real life example of what the latter can do is in Alexander the great, who spoiler alert: named like everything after himself, and had no plans for the rest of the world beyond his own lifetime, which resulted in the immediate collapse of his own empire, and the death of countless millions.
No, it's more likely that he's not inbred, he's just a cunt.