Jon was willing to accept the surrender of Cersi’s army, but they had in fact been following Cersi all along. Cersi showed that she would not have given anyone quarter if she had prevailed. In Lonesome Dove we learned that if you ride with horse thieves you will hang with horse thieves.
Except by the time those bells rang, the Golden Company mercenaries were completely destroyed, and the Lannister troops in the city had thrown down their weapons. The actual fighting was over. She literally had no soldiers left other than a half-dozen Queensguard. There was nothing left to stop the UnSullied from marching right into the Red Keep and grabbing her.
Even if she though she didn't want to take any chance that Cersei might be left alive, the most she had to do was take her dragon and destroy the Red Keep itself. But instead, she took that dragon and literally targeted all the civilians and private homes between the walls and the Red Keep. There is no conceivable moral justification for her murdering over a hundred thousand civilians given that the fighting was over.
I do see your reasoning, and throughout the series we have seen people alter their perspective. Most recently Dany becoming less reasonable, but what pushed her to this position? There were many chances given to Cersi to save her people. And Dany too had saved people many times. But the circumstances of the final battle had to pay for all their faults.
I can see a leader deciding that giving quarter to a side that would give no quarter does not make sense. I agree that Dany could have been more sensitive to the collateral damage that she was doing but I cannot fault her for making her decision. In this case I want to respect her decision. We will see what the writers end up saying next time.
Perhaps this was showing what kind of a leader she would be in order to give us a direct comparison to the leadership we would have with Jon. Although we do not get to see how Jon would have handled it. Even when the swords were thrown down, Jon was hesitant as to what they should do next.