So speaking as someone who’s never watched an episode, if the bells going off is what triggered her and made her randomly snap...why did she not then immediately go attack said bells instead of flash-frying a bunch of rando civilians?
The bells did NOT make her snap. That was a bad description in a batch of leaks that, although in hindsight true in nature, were snippets partial at best and lacking context. Most likely the leaker was either on set and the bells were the cue for the physical effects going off or somehow involved with the editing and managed to have a look at that segment.
I premise the following by saying that I consider myself to be a book-snob, I gloat at the ragefest the series is turning into and at the general mindless backlash of the GA and I hope the hacks D&D career will come out utterly destroyed.
What I gathered from that scene (and a debate can of course be had, on whether it succeeded in conveying it) is that the city has surrendered, Dany has won without bloodshed. And she does not like it. One episode prior she stated that she was not interested in parlaying a peaceful solution for the war, but it would however been useful that people saw her good faith in trying to negotiate with Cersei. So that the population would know whom to blame, when fire would have rained on them from the sky.
A prior scene in the last episode also established that Daenerys has finally realized she is not loved by Westeros. So she will have to resort to rule by fear, and that's it.
So now they surrendered, she has won, and it is not as rewarding as a fire-fueled open battle is to her. People are not blaming Cersei for the misery befelling on them. These ungrateful vermins must pay for it and set an example for the rest of Westeros. More generally, this is shattering her childish notion of herself as the beloved rightful queen people are waiting to cheer on and reinstate on the Throne. She is not the rightful heir, she is not beloved, she is not the savior she convinced herself to be.
This is the last straw on the progressive destruction of the entire mendacious identity she built for herself her whole life.
As I said in my last post in this thread, however, apart from the rush in the storytelling, the audience is reacting so badly because HBO, the showrunners, the conniving 'critics', the SJWs always portrayed Daenerys as a strong independent woman who can't do wrong, and the audience acritically lapped it up. Now they are crying about destroying her character and her arc (lol) for the sake of 'subverted expectations', and one way to cope with that is being snarky about the bells making her going bonkers, while it is still the same rushed, shoddy, hacked way of storytelling the last seasons had, courtesy of D&D, and that was praised to heaven by the same people now screaming bloody murder.