When it comes to having personal experience with a difficult thing, there are two types of people:
A. People who are forever changed by their experience and become more compassionate about other people having to go through it
B. People who gloat at, mock, or contribute to the suffering of people in that situation
You might go "huh?" at this but think of it as the Senior Phenomenon. In high school, it's very common for freshmen (or whichever class is the youngest in the school) to be viciously harassed by the seniors (or whichever class is the oldest in the school). The seniors treat it like it's their right to pester and torture the youngest class, and freshmen who complain about it are often laughed off as if it's just a rite of passage that they are required to suffer through.
Some freshmen think this is obscene, remember how awful it was when they're older, and choose not to contribute to it.
Some freshmen can't WAIT to do it to others when they themselves become seniors, and derive a great deal of delight from torturing the next generation even though they aren't hurting the people who hurt them.
I understand (though don't personally engage in) actual straight-up revenge. If someone hurts you, sometimes you want to hurt them--either to make them have to experience something negative because they caused you to, or to teach them a lesson. But this kind of weird delayed "revenge"? I don't get it at all.
Here are two ways I've seen this manifest in my real life.
I know someone who used to be fat and now says casually fat-shaming things a lot. When I have called them out in the past, they have consistently defended their behavior by making reference to when they used to be fat and suggesting that frees them of the responsibility of being sensitive about their language. If they say it, and THEY used to be fat, after all, how could it actually be offensive?
I know someone who used to work in a restaurant and now goes out of their way to torment servers. When I've tried to counteract, apologize for, or request a change in their behavior, they frequently say something like "Look, I did my time working in these places, now it's their turn." Knowing how awful restaurant customers can be, they still knowingly "test" them, ask for special treatment and comment on the server needing to hustle for a tip to their face, try to get free food by implying the server needs to do back flips to get a good tip, and even sometimes sexually harasses them. Since they had to put up with it back in the day, how could this be anything worse than good clean fun?
If I'm honest, I hate this more than I hate obliviously inconsiderate people. At least with people who have never been in those shoes, you figure it's ignorance--they don't realize that what they're saying or doing could be hurtful, obnoxious, or just generally ugly. I feel like there are a lot of people out there who have to be personally connected to an experience before they have any sympathy for it--like all those people who bluster about how you just need to suck it up and pay for medical insurance until they or their family member ends up uninsurable, and then suddenly they realize the system is broken. You really shouldn't HAVE to have personal experience with a situation before you can realize you're being callous or actively hurting them, but I can at least see HOW that can happen. Because other humans are an abstraction to a lot of folks who want to avoid feeling guilty about those they're harming (or not helping), so it's easier to blame strangers for their own misfortunes than it is to stop making their lives worse. He should just lose weight if he doesn't wanna get harassed. They should just get a better job if they don't wanna deal with difficult guests.
I used to work in retail. I was not a jackbag to retail people before I had my turn on the floor, but working in a store for six years gave me additional insight into what customer behaviors make life difficult for retail workers and what the employees are handling when I'm not looking at them. I used this experience to further inform my behavior in public while interacting with these workers now, and I will even sometimes inconvenience myself to avoid inconveniencing them. (I don't expect the average person to do that, but I hope that they wouldn't deliberately inconvenience them.)
I cannot imagine, after walking in those shoes, feeling justified in ruining current retail workers' day or contributing unnecessarily to their workload. I know that most employees genuinely want to do a good job, do not become more willing to help me if I vent my anger or disappointment at them, are not personally out to cheat me, and aren't in danger of losing their jobs if customers stop making messes. Honestly, the "I'm GIVING THEM A JOB" attitude some people have when justifying their wanton destruction is so entitled and obnoxious. The number of hours I was given on the clock were not affected by how much customer cleanup I had to do, so self-importantly defining yourself as a provider of job security as you inconvenience me is vile. And to get back to my original point, it reminds me a LOT of the people I know who engage in fat-shaming and restaurant worker abuse.
They feel they have earned the right to hurt other people because of an experience they've had, and it's pretty gross to me that they find hurting others rewarding. Why is it delicious to them that someone will suffer? And when someone (like me) suggests this is bad behavior or may be evidence for them being a terrible person, they are so very quick to suggest their behavior is ultimately helpful, despite it lacking empathy and making the world a worse place. Fat shamers frequently suggest their behavior is contributing to a healthier existence for the fat person, and people who abuse service workers frequently suggest they're incentivizing the employees to move up in the world. I promise that these folks do not need your personal help to make changes in their lives and they do not appreciate "help" that comes in the form of making their lives awful.
Making a space or experience so awful that people will be desperate to escape it is not a healthy way of "helping" anyone, and furthermore, it's a transparent lie that you're doing this for them.
You're doing it for you, because it makes you feel good.
For some reason, the way you support your belief that you've moved up in the world is to abuse people who are currently where you were, and you then convince yourself that this is happening to them because they deserve it.
That's toxic, unhelpful, and bad for everyone, including you.
I try, but I fundamentally don't understand why tearing someone else down feels good to anyone. I realize, after living in the world for forty years, that it does accomplish this for some folks. People revel in the suffering of others far too frequently for me to suggest that they aren't really enjoying it. But if I hurt someone I didn't mean to hurt, it ruins my day, and if I have to do something that I know will hurt someone, I will look for ways to avoid it and do it reluctantly if I have to. I don't relish the experience and I do my best to make reparations (not excuses) if it's in my power to do so.
But for people in the "B" camp from my initial description, something different is going on. I think it must have its roots in selfishness; those people don't care who they hurt if it isn't them and theirs, and if the pain they've caused arrives on their doorstep and forces them to look at it, they'll usually do one or more of these three things: a) blame the victim for doing something that naturally caused the hurt to "happen" to them; b) deny that the hurt they caused was caused by them or that it hurt at all; or c) claim that their action should not hurt the victim, despite that it was designed to do so. In other words, once they cause pain and it's humanized, they have to jump into distancing themselves from their actions somehow.
Especially if you've been in a certain pair of shoes, really imagine what it would be like to be in them again, right now. You're fat and someone calls you a gross name and laughs at you. Now, really. Realistically. Did that motivate you to lose weight? Was it that person's business what your weight is? Do you now feel like you owe it to society and yourself to be thinner? Are you convinced that the sneering, guffawing person screaming at you from a car did that for your health? You're working at a restaurant and someone changes their order but accuses you of just remembering it wrong, and then shames you for contradicting them because the customer is always right and you should be working on your tip. Do you sigh and nod and understand that unreasonable, lying people out to blame you for their mistakes are inevitable, impersonal speed bumps in this trade, not individuals making choices? Do you just shrug when the gloating customer stiffs you on the tip and accept that this is your just deserts for failing to kiss ass properly and go home to put in applications for a "better job"?
You probably don't do these things when people abuse you. You probably just feel depressed, hopeless, and helpless. For the record, studies show that berating someone is far more likely to result in self-hate and withdrawal from positive engagement. It does not often motivate someone to make changes they weren't previously inclined to make. You can find an occasional story where a fat person snapped after one too many comments and ~decided to get in shape~, or a restaurant worker was abused by one too many unreasonable dinner guests and ~decided to start their own business~, but that is not to say these people owe their success to unkindness, and it is also no excuse for those who choose to dish it out.
You can help through compassion. Be better than that. Don't be the senior who dumped their lunch on a freshman--the person that freshman remembers for the rest of their life as a cruel, laughing asshole who taught them nothing and made them want to vent their impotent rage onto a future generation. You aren't helping them, and you aren't helping yourself, if you choose this kind of "revenge."