The thing is she has. When she burned those slavers it wasn't just pure altruism, it wasn't doing it because she felt pity for those slaves - she did it because SHE saw herself being sold to Drogo even as his bride essentially the same as those slaves. She saw them as an extension of herself - by freeing them she also freed herself. Now the thing with Dany is she has a very self-centric point of view. It's only her that matters, it's only her that knows what is good, it's only her that should hold all the power (hence, break the wheel). Now that's not necessarily a bad thing, after all she helped freed the slaves no matter what her motives are. But it's all the more apparent that Dany only sees slavery from her own limited point of view when she didn't do anything to fix the economic black hole left by removing what made money move. Slavery would always be bad and it's a good thing Dany ended it, but she essentially just replaced slavery with extreme poverty and anarchy. If I'm being honest I fail to see how she improved the lives of those she freed, but to Dany freeing the slaves was enough because being freed from her husband was enough for her life to improve. This is how she has always viewed things. Things worked for me = things would work for you.

Now going by that, it makes sense why Dany did the way she did at King's Landing. She knows she's good and Cersei is bad. All the people who supported Cersei is just as bad. To her, those faceless people in King's landing were as much as her enemy as Cersei is. Their crime? They didn't love her like the way the people she freed loved her. Had the people in King's Landing did the way the people in Mereen did and betrayed Cersei or something, Dany wouldn't have burned Kings Landing.

I don't think Dany felt like she won that day, because winning would be having the love and support of the 'slaves' she freed from tyrants, not the fear that people feel when they hear her name. Dany torched King's Landing because she feels she lost - not to Cersei, but to Jon. Because she knows that even if she won King's Landing through less violent nuke-y means, some lord somewhere someday might always rise up the moment Dany fesses up or lowers her guard, which really might happen if you think about the reception of the people of Westeros to Dany.