I don't know why so many people are upset by it and saying it's "out of character." So is murder, but tons of people do it anyways. No one is murderous by birth. It's new and it can happen to those who show no tendencies of it.

I think people are just upset by the fact that she was so just and fair in every season, but the reality is - she's been "just and fair" for simply material gain: ruling the Iron Throne at all costs. Even by her own admission, it's all she could see.

So why is it surprising she went ape-shit at Kings Landing that cause her so much pain and hardship and to top it - killed 1 of her dragons? She viewed even the people in it to be supportive of Cersei, not innocent hostages like Tyrion claims.