People troll others for all kinds of reasons. My job makes me have a more public persona than most and I depend upon the general public for my livelihood. Although I'll get the occasional rude comment about my actual work (someone didn't like it or they have a valid criticism or complaint) I also get 10-20 trollish comments every week from people who are literally just being mean. They'll write me and make fun of my hair, criticize my looks (which have nothing to do with what I do for a living), make fun of one of my features, etc. Sometimes they'll cyber sleuth me and find things about my kids and make fun of THEM. Trolls troll because they enjoy getting rises out of people. That's what they do and it's why they do it. No other reason. And I absolutely believe that the Harts got obnoxious comments from people over that picture-not honest criticisms but people just being mean. A regional nightly news program did a story about me a few weeks ago and I am STILL getting messages from people, once again, making fun of everything from my looks to my house. I'll occasionally get the "you're so ugly you should die" remark. (And, excuse my conceit here, but I am far from "ugly" but even if I was, it's just cruel.) I once got a message from someone who had found a toddler picture of my daughter with Santa Claus. They said that they had shared it with a pedophile group and that the consensus was that she was, "An abomination but still rapeable." That was on Facebook. I reported it and FB closed that person's account, but I am sure they opened up another one within minutes. While people DO generally move on and turn their attention elsewhere, there's always someone new discovering the story.
There are those who get angry for valid reasons (they don't agree with what the person in the story was doing, they see an injustice, etc.) but just as many lash out in anger for reasons that don't make any sense at all (or they're just jealous because someone else got attention). It's really hard to say but it happens a lot.
And how do people find folks? I guess the same way that we do on here-they sleuth.
All you really need is a first and last name, or else a first or last name and town of residence. With technology and social media being what it is, folks are easy to track down these days.