Dany made a choice.
She did not snap as many have said including the DDs. The bells did not trigger her, it was simply the point when we all realized her premeditation. Why then did she make this decision to kill the innocents? Since there is no love to help her rule, she must use fear. Kings landing isn’t the only place she must conquer. There is the rest of Westeros she must deal with. Especially after Varys sent all his letters which she probably knows about. Murdering here may save lives later. Similar to the US in WW2 after beating the Nazis, the US then dropped the Atomic bomb on Japan. Japan still didn’t surrender and the US did it again. Killing 300000 civilians. Through the “excessive” use and fear of the super weapon, an estimated 1 million us soldiers and 3 million Japanese were saved from a US military invasion of the country. Emotion played a part for sure but this was a strategic decision which she believes is merciful in the long run.