Criticism lies on the issue that they showed us there was something wrong with no appropriate build up at all, disregarding her entire development done over 8 years.
That's not what they were conveying. Remember the Varys scene, and the analogy that mentioned when a Targaryen is born, a coin is tossed and the whole world holds its breath (paraphrasing)? He was implying that she might be mad and everyone would suffer, before dying he even wished he was wrong for the good of everyone.
Turns out she was mad, Varys was right, and that's exactly what the writers were conveying to the audience. The same kind of mad her father was. Burning innocents is not a "rational and calculated manner", she had an emotional breakdown when the bells rang, how could anyone possibly say that was calculated and rational move? We may even see her having an emotional struggle filled with regret of burning all of those innocents next episode. That or just full blown mad, no fucks given, we'll see. Sadly at this point, I've just given up, hopeless.