I think the basic idea she's trying to get across is that people feel helpless in our daily lives, and so the one little piece of power most people ever get is online and we take out our frustrations about our powerlessness here.
Which probably says more about the people who get a tiny bit of power than it does about the Internet, but there you go.
This story belongs to a greater category of stories with headlines that start with the word "Why..." promising an answer but never delivering. They're very nice stories. Very descriptive and well-written. But the headline is a bait-and-switch. The honest headline here would be, "Why Are People So Awful Online?" but the honest headline wouldn't get as many clicks. I hate the Internet.
Just so you know, even in print media the author does not generally pick the title. The title might tell you something about the general subject of the essay but you shouldn’t take it as a precise metadata. The predates the internet. Start reading based on the general topic. The first few lines (the “lede”) will tell you if you want to read the whole thing.
It hypothesizes that we respond to petty wrongdoers the same way we'd respond to war criminals, and that we further have been corrupted by exposure to too much bad news.