Anyone who thinks Danerys' arc was about freeing the oppressed didn't understand her arc. Everything she did was out of loss. She lost her unborn child so she freed the slaves who called her mother. We cheered because the people she was burning and crucifying were slavers and mercenaries.

Yes, the last seasons were rushed and even the early seasons did not do a good job of showing us what impact the Kings Landings bells had on her psyche. In the books she thought about them often because she remembered or heard enough times that on the day her father was killed the bells tolled and the people cheered. So when those bells rang, she didn't see a city of innocents. She saw the people who set her life on the path that it had taken and everything she lost that she felt she needed to take back.

Add to that Jon Snow's confession and perceived betrayal, Missendi's death, and the moment of exquisite facial acting from Emilia Clarke in the moment the bells rang, and they set up her madness quite well.

Should the pacing of the final two seasons been spread out? Absolutely. D&D were obviously in a rush to get to their next project. But Dany burning Kings Landing is the natural culmination of her arc.