In rewatching the scene where the bells sound the surrender I think I understand a bit more of how they envisioned the character motivation. Note that Dany looks angry as she's sitting on Drogon's back after having blasted a ton of shit but once the bells start to ring she gets REALLY pissed. Emilia Clarke may not be the best actress on the planet but she's good enough to convey an uptick from angry to completely livid.
Dany is pissed because they surrendered. As mentioned at the top of the episode all the other citizens of cities she's come to the people have risen up to overthrow the masters, but here these people in King's Landing, the same people who stood by when her father and family were murdered, do the same now. They don't rebel against Cersei they just cower and expect that by causing all this pain and grief they now get to surrender and say 'okay we're done, no harm no foul amirite?' In that moment she decides 'fuck these people, fuck these Westeros entitled fuckers' and lets it all just burn.