1. Cersei has Qyburn address the public, making up lies and vilifying Daenaerys telling them that she's riding south on her dragon to burn down the city and everyone in it just like The Mad King wanted to do. 2. Qyburn gains Cersei some public favor by promising to protect the city and everyone who lives in it by showing off the ballistas that were built to defeat the dragon. 3. Missandei gets captured, and then the meeting we see with Dany and Cersei takes place just like we see in the show... except... 4. Instead of Cersei just executing Missandei. She is instead lead to the center of town and tied up in front of the entire city. Where men, women and children all take turns throwing stones at her all while they laugh and cheer. Which would have made so much more sense when Dany starts to slaughter them all.
Think back to the scene from Star Wars: Attack of the Clones.. and say what you want about those movies but this scene is powerful. It's the scene when Anakin goes to find his mother and he finds her tied up and dying in a Tusken Raider camp. So when his mother finally dies and he becomes so enraged that he slaughters the entire camp, that makes sense to me. That's why I think they should have found a way to justify the animosity towards civilians.