Did people forget about Dany's look into the future in season 2?
When she was in the house of undying she saw a vision of the future, she was in the throne room and it was all in ruins and burnt with ashes everywhere. And Bran had a vision in season 4 where he saw the shadow of her dragon flying over Kings Landing. What she did really shouldn't be a shock, the entire show has been building to her becoming her father and burning everything.
I know alot think it was just so sudden without any proper build up, but the build up was there all the time.