I interpreted Batman’s comment that Barbara “hasn’t been to the edge of the abyss” not as justification for her assault, but as foreshadowing of her taking down and almost killing Paris out of anger. She saw what she could let herself do now that she had someone she cared about, and it undermined her crime-fighting.
I liked the movie overall, but still trying to reconcile the two parts; Barbara’s introduction and the Killing Joke. I also wonder if the Joker actually raped Barbara or just wanted Commissioner Gordon to think he did (or wonder if he did). It’s still horrific, and she was still assaulted, but I thought the ambiguity was an interesting touch. Uncomfortable and dark material, but I thought it worked.