It sucks that stuff like this happens, but very few of us can say that they never take part in this kind of shaming. I certainly do on occasion. She even said she did. It's just kind of human nature, as bad as it is. Which explains the shock people feel when they realize that every picture, every comment, every post on the internet came from an actual human being. What I think is funny is that, personally, this picture isn't near what comes on up from some people. It's a girl in a costume for Halloween, and she probably had fun that night. It's really not that bad, and it got such mean comments. I cannot imagine what thoughts someone has when a picture goes viral of them having an accident or taking part in something they're not proud of. The comments could be absolutely destructive to an already teetering self-confidence.
I think you're right that most of us would have chuckled at and maybe even shared a photo like this, but the hateful comments are a lot rarer to common and decent people. Stuff like "Heifers like her should be put down" is just so monsterous it takes a real asshole to post something like that in public.