It's a pretty fascinating tale. His reign as king actually started out pretty good. But as his wife kept having miscarriages and his relationship with his hand, Tywin, deteriorated, he began to lose it more and more. And THEN he was kidnapped and held prisoner by one of the powerful families in the realm, who planned to execute him. I think that was really the point of no return for him. He became super paranoid, didn't trust anyone around him, didn't leave the red keep for years. That's when he started burning his perceived enemies and traitors, and feuding with his eldest son (Jon's daddy), and then eventually killed Ned's father and older brother, kicking off Robert's Rebellion.

He wasn't always the mad king, his descent into madness was a gradual one. Not like Dany's 3 episode swing.