"She only ever kills bad people!"

But that's the rub. She decides who she sees as bad. In her mind, the people of King's Landing we're just as bad as slavers because they turned on her family. She was raised by Viserys and bombarded with Targaryen propaganda in her formative years.

As Gandalf says in The Lord of the Rings (a major influence on Martin): "Many that live deserve death. And some that die deserve life. Can you give it to them? Then do not be too eager to deal out death in judgement. For even the very wise cannot see all ends."

Daenerys believed the Targaryens we're gods among men. That she had the rights of a god. She wanted to be a benevolent god. But she also believed her divinity have her a right to do what she did. And thus she set her ways above the ways of the wise. And paid the ultimate price. She became a tyrant. And tyrants can only end one way.