I think the writers rushed in to making her evil. She was fine until one by one all her trusted advisers were killed off. They tried to show that even though she won, she didn't get her fill of the bloodshed so she just went around burning the city. They could've set a trigger, like a face-off with Cersi, which would make it almost justified but too much at the same time. It would make more sense that the nephew fucker, errr excuse me, Danny tries to burn Cersi but isn't able to so she starts with the castle.
On a different note, what kind of fuel does the dragon use? That thing doesn't need to refuel at all just goes on and on flaming all over the place. I was like, take a breath your throat must be sore by now.