It's simple: to feel power over those who hurt you. Making somebody who made you feel powerless and worthless feel the same way feels so satisfying (at least it does for me).

For example: back around 5-6 years ago, I was at a votech school. There was this girl there that I had a crush on. I befriended her and was trying to get close to her and I added her on Facebook and what not. She had a high IQ and was an ambitious overachiever. She was also a bully towards people who she deemed less intelligent. Because I had poor social skills, she often treated me with contempt. She also had the school's backing because she did so well that the school used her for advertising purposes and what not. Needless to say, she later blocked me on Facebook and stopped associating with me. After that, I went to a major gaming convention and met a YouTube gaming celebrity that I idolized back then and got pictures with him and $150 worth of free gaming stuff from the company that sponsored him. I used my newfound fame and made friends with half of the people in my class since they were also heavy gamers. I used my newfound popularity to make her feel worthless and later sabotaged her reputation a bit which cost her a job that she was trying to obtain. When she blocked me, it made me feel so worthless that I was feeling depressed and suicidal for a whole week. After making all of these friends and meeting that famous YouTuber, I never felt happier in my entire life.

Having power over others who used to have power over you is the best feeling that I have ever experienced. Most people would disagree with me but it doesn't matter. No two people are the same in this world so there isn't a one size all approach to treat the human condition.