He’s tall, handsome, and next in line to the throne. He’s got it all and he knows it. Why should he be nice?
Let’s give him the benefit of the doubt at the beginning. He’s spoiled and privileged. That doesn’t mean he’s intrinsically awful. When he rides out with Sansa he shows her a lovely day in a beautiful countryside. It’s only with Arya that his dark side emerges.
For a while he seems like a rich boy, petted and selfish. Underneath, though, is a liking for cruelty. He’s not that different from little Robert Arryn, the breastfeeding lordling. He wants to see people fly through his own version of the Moon Door just because he can.
Benefit of the doubt officially over.
He’s a monster, but here’s what’s interesting. When he becomes king he immediately behaves as a tyrant. There’s no warm-up, no testing the waters of power. It’s not politics for him. He just is this way. The only difference is that now no one can tell him to stop. Before this he seemed possibly juvenile. But he’s a sadist. He enjoys the pain.
He’s passive until the throne is brought to him by his father’s death. He doesn’t engage in politics and he makes no effort to learn. When he sits on the throne and rules, cruel whim informs his decisions. Law and justice mean nothing to him. He’s no Head Type.
And he’s no Body Type. Little Arya beats him up. Weapons training is not something he pursues with passion.
So, Heart. Two, Three, or Four?
He’s not clever. Sansa talks rings around him. So does The Hound. He’s not nice enough to be a Two and not successful enough to be a Three. (His mother got him the throne. And no one likes him there.) That leaves a nasty Four. That explains his ability to misdirect people with his prettiness. Smoke and mirrors.