Well I disagree, during this season and the episode she started to see all of Westeros as her enemies. She was threatened and felt betrayed by everyone, she even said she chooses fear. She was also hungry for power and wanted everyone bow down to her. She was ready to burn the place down if Cercei didn't surrender and Cercei didn't. It didn't matter to her anymore when the bells were ringing, Cercei didn't bow down but killed yet another of her friends.
Her going straight to Cercei would not have been in line with her character anymore. Maybe a season ago before her arriving at Westeros but not anymore.
She chooses fear and wanted everyone to bow down to her. But then she killed everyone, so no one is left to fear her or to bow down. Stupidest move ever.
She wants, since the beginning, to be Queen. She wants to rule. For that, you need people and a kingdom to rule. If you destroy everything and kill everyone, then you're not a Queen or King, just a murderer. Huge difference between "killing and being ruthless to become what she wanted" and "killing and destroying what she always wanted because her nephew wasn't in the mood last night". And of course, let's not forget that she completely ignores her enemy, letting Cersei escape (or the possibility to escape at least), because it's obviously more important to kill innocent kids in the street for 30 minutes. The writers butchered her character development and this whole sequence was embarrassing. If they really wanted to make her a Mad Queen, they should have added 2 or 3 episodes to show how she would evolve like this. Oh, and they should also have learned to write a story, or hire an other writer (a competent one) to write that part. They just don't have the skill for anything deeper than dumb butchery.
EDIT: She could even try to kill Jon, it would have been a lot smarter and more logical than what they made her do.