Originally Posted by Excelsior.
That was a horrendous writing low point of the film. It's the purest violation of the show don't tell rule. The entire turning point of her arc is reduced to a single line.
To be fair, they did show it before they spelled it out, during the montage. You can see how tepid and unsure she is while everyone is partying in the mansion. That line just cemented it.
I also think her real turning point came prior to that, in her reaction to the Bane beatdown of Batman. She realized she had aided a true monster.