GRRM puts a little bit of "madness" into every single Targaryen, even the good ones. Just like John Brown was a little bid mad. It still is definitely not true to Dany's character to have her become a tyrant for no reason. When you are in her head for thousands of pages it's more clear how completely bullshit it is.

The show was much more full on cynical, which matches the "trying to fix things is evil actually" cynical centrism of HBO and the showrunners. The books are not like that really, they display terrible stuff happening but they're filled with hope and idealism and heroism. Things are not futile here, history and people can be changed and improved and society can evolve.