I inadvertently slapped a woman's hand before. And, she really wasn't doing anything really wrong. Just being sweet (in a bit of a loud way) around my daughters when they were younger. It was like a nerve reaction, I didn't intend to do it.
You're going to get opinions all of the time. It's difficult when they're crying in public places because you know your child is being disruptive as well. My first had a crying schedule, 7pm - 9pm on the dot, every day. (she's 14 now). During those days, my mother would call from the E. Coast at about 8pm my time, 11pm her time because that's when long distance rates went down. But, we couldn't talk because she was always crying.
Honestly, rather than deal, just schedule around it. You're aware of her schedule, try to find a way not to be in public if you can when she gets like that. I guess for me that was easy because it was late at night and her bedtime anyway.
But, if you do lash out, don't feel so bad. Blame it on your hormones & lack of sleep.
S.