I read somewhere that she was frustrated by how easy it all was. She had let her advisors convince her when she had a full army and 3 dragons to go help Jon Snow with his wight capturing and white walker war BEFORE going for kings landing. She lost two dragons, Jorah, half her army, and Missandei because of it.
Even after rude af Sansa told her that her troops needed more rest (try to tell me Sansa wasn't stalling to play more politics TRY ME!!) Dany still EASILY took KL. She wanted this to be the fight of her life and she just took it. &&&&& Jon Snow was any day going to come in and take her throne (because let's be honest he lets his sisters rule him way too much and already "for the people" Varys had tried to poison her) so why give mercy? Why not rule in absolute fear?
Dany has come in and conquered. She tried to peacefully in Mereen and that taught her fire and blood.
Is she a Ned Stark? No. But she's certainly not mad. Throughout the show they proved how she wasn't mad, Targaryens don't flip a switch. They are born mad or not. Dany developed into this. Which I think is a little deserved lol.
Also I think this is himym 2.0. Did GRRM/DD know how this was going to end all along? Yes, they did. But as the story has evolved and what it has become, was it the best route? Probably not. There are going to be a lot of angry fans because the writers didn't realize how liked Dany was going to be. How especially in this political day and age, a strong, confident woman (since day 1 I might add, none of this "my rapers are the reason I'm strong" Sansa bullshit) is what the people want. && now because of predetermined storyline, her reputation is going to be decimated. Well I for one as a Dany fan say FIRE AND BLOOD BITCHES!!!