Joffrey is such a massive prick, to be fair most of the characters are real shades of grey, but have some sort of redeemable quality hidden amongst their personality somewhere. Joffrey is just a straight up, 100% massive cunt.
I think everyone's a sort of "grey" because you understand why they do what they do. You have clear motivations and reasoning. They're doing something that may not be so good because they had Y and Z things happen to them. With Joffrey it's kind of the same, except that those Y and Z things are his parents. His upbringing is his main motivation and reason he's such a cunt. He's clearly a product of both his mother and his father. His father has no respect for women, and less for his own wife, so obviously Joffrey isn't going to really grow up with much of that. His father also always got what he wants, so that's how Joffrey sees kingship. At the same time Cercei was kind of bringing him up as a sort of revenge to Robert. He was her hope toward the crown and the power it holds, since she held not much sway over Robert himself. She raised him to know he would one day be powerful and could one day get whatever he wanted. Except she kind of counted on being able to manipulate that more. But, again, he takes after his father in that way. He won't be manipulated by her. So his abhorrent treatment of Sansa and his disobeying of every adviser around him is a product of all of that. He never really gets any consequences for his actions before becoming king, so obviously he isn't going to think any of that through while he is king. Think of when he got in the fight with the butcher's kid and Arya. The kid and the wolves were killed. That's exactly the consequences that he's making happen as king, too.