Yes, she wanted to destroy King's Landing from the get-go.
The Tarleys would not swear loyalty to Daenerys and said they would stay loyal to Cersei ("we already have a queen"). They either needed to die or go to prison, but could not be let loose to do more mischief for Cersei.
Daenerys has never been flawless, but up to now she was portrayed as "more sinned against than sinning".
I’ve seen her megalomaniacal tendencies for a while.
They always think the way from where they are now to utopia requires spilling an ocean of blood. So they do spill an ocean of blood - but utopia never gets any closer. She claims “future generations” are why she’s such a butcher and that they’ll never suffer a tyrant’s rule with her victory. Firstly it looks like that future generation is going to be a lot smaller since she killed so many people in the present. Secondly, what tyrant could be worse and could do more harm than her?