"The more the better"
Thi is the approach the writers took on this one.
So these writers sat and said:
- "You know what? People love to be shocked. What can we do to shock them even more? I mean, we have already beheaded our seemingly main character. Then we slaughtered our new seemingly main characters while stabbing a pregnant lady in the guts. And then we burned a little girl on the stake. I mean, what could we possibly do that would top that?"
- "Well, we could have Dany attack the Red Carpet, killing in the process hundreds of the civilians taking shelter in there"
-"Nahh.. We NEED MORE"
- "Ok, how about after ringing the bells and the city surrenders, Cersei gives the order to activate the traps with wildfire, killing most of Dany's troops, causing Dany to completely lose it"
-"That sounds cool. But not shocking enough. We NEED MORE"
But it turns out the more is not always better. Just like you, I was totally expecting Dany to snap, but when it happened I was like: "Really? this is the best you came up with?. Sorry but to me the whole scene seemed so cartoonish that I just couldn't stop laughing at it.
Nah man, I don't like or watch Transformer movies and stuff like that. I thought it was good writing that she snapped after the city surrendered. When I watched and the bells rang I was a bit baffled that after her scenes with Jon and Tyrion she would just take the city without killing civilians. She was obviously going crazy and Cercei did not surrender. She did threathen to burn the city down.
What I liked is that when she started killing they didn't show her anymore. Even though I was expecting her to snap it left the viewer as stunned as Jon, Tyrion and even Cercei. They were predicting it but hoping for the best. We don't know what's going on with her and what happens next.
Your examples might have worked too but it would be a little straightforward. It wasn't supposed to be yet another moral choice for Dany to accept civilian casualties among enemy soldiers, we've been past that. And Cercei with another surprise trap..meh