I have no idea why people haven't sensed since season 1 that Daenerys is quite selfish and self-important, and besides being exiled she have been given everything in a silver plate compared to the characters in Westeros. And for me it was quite obvious that she would be a tyrant in the future. And it showed obvious on the show when she discovered of Jon and couldn't accept to not be queen by being married to the king, it should be her the all important.
Anyone denying that being impossible to burn and having 3 dragons given away is having most of what she needed to conquer what she had is wrong =]
The issue isn't that she did what she did though.. it's the fact that this was the equivalent of a sudden pro wrestling heel turn with no story defined rhyme or reason. It essentially boils down to "I screwed up and let my second dragon die and my lover didn't kiss me back soo.... off I go doing everything I swore I would never do."