Daenerys was a deeply traumatized woman who had to endure tons of abuse.
What gave her the strength necessary to survive all that was her strong belief that she was destined for greatness. She learned how to cope with her pain by telling herself a story and believing in it.
Everything she did on her journey was based on that. And she had a lot of success. Her belief grew, and the people started to see her as a force of destiny as well. She tried to achieve her goals by being merciful. By helping others. By fighting the bad guys. She hoped that once she reaches her goal, the Iron Throne, that everything would be fine. That her pain would vanish.
But it didn't.
She experienced further loss and rejection. All while having the power to just take what she wants with violence.
Daenerys didn't become mad. She didn't become evil. She became angry when the bells rang, when she had achieved her goal - but the pain still remained.
That's also why she decided to continue her conquest even after the destruction of King's Landing. In order to "free" the entire world. Her journey gives her strenght and comfort. She can't stop. She has to be the master of her own story, and that story cannot end.
I don't see Daenerys as a villain. Her life, her fate is deeply tragic. She's another broken human in a fundamentally broken world.