If you've ever been a kid (which is everyone) you know that kids can be brutal. They have no filter and a desire to hurt. Sure, people can be mean all the way into adulthood, but nobody's meaner than a 5th grader with something to prove. The scars left by elementary and middle school bullies run deeper than most wounds. They get you when you're young and vulnerable.
Those of us who remember our childhood tormentors often dream up scenarios of what we'd do if they could turn back the clock. We would stand up for ourselves, have the perfect thing to say, and put them in their place. But nobody (to my knowledge) actually has a time machine, so we're left to let our wounds fester and feel a twinge of bitterness whenever we recall the memory. But some people are cunning, and exact revenge when they have the chance. Like in this recent Reddit story where a bullying victim pulled off the ultimate prank to get her bully back.