I meant only the things I am aware of. (And excluding the category of people who deserved it.)
Simply said, if someone hurts me, it means the person is an asshole. If I hurt someone, it means I am an asshole. The latter fact is more important to me, because a) I care about myself more than about other people, and b) I will have to live with myself for every day of my life, while other people will come and go.
Once I hurt a random bystander, because I wanted to fit with a group of cool kids, and they decided they didn't like his face. Yep. I am not the kind of person who does this often, and I would want so much to say that I am the kind of person who would never do it... but I lost the right to say those words since that day, irrevocably, forever.
And what did I gain in return? Pretty much nothing. It didn't win me a group of cool friends, mostly because those kids soon stopped being friends with each other, and some even turned into bitter enemies. Using the wisdom of hindsight, many of them were a lousy friend material in the first place. A few years later, life took us in different directions, and I haven't seen them since. These days, finding and contacting them using social networks would be trivial, but I just don't care anymore.
If I could turn back time and use the wisdom of hindsight, I wouldn't give a fuck about the cool kids, and would say "hello" to the stranger instead. And, who knows, there is a chance I would have found a good friend. -- But now, whenever I notice I betrayed my ideals again, I remember that day as the symbolic moment when I wandered off the path of righteousness.
Compared to this, situations when other people hurt me seem less important, often didn't have long-lasting consequences, and sometimes even turned into something good! For example, a girlfriend cheated on me with my friend, and a few other mutual friends knew about this, but no one said a word to me. Then I got some evidence, and after connecting the dots I realized they all have been making fun of me for quite some time, making hints to each other I didn't understand. That hurt!
So I broke contact with all of them, and drowned my sorrow in workaholism. A few years later, I had more money than most of my former friends combined. Then I burned out, so I decided to slow down and enjoy life again, made new friends (much better ones), and new girlfriends, and lived happily ever after. :) One day, I met my former friends, we had a beer in a pub, I watched them and listened to their stories, and realized they were a bunch of losers, and they unknowingly did me a great favor by preventing me from becoming one of them. Never met them again; don't care.
Which of these stories was better, speaking from a completely selfish perspective? The one where I hurt someone, or the one where I got hurt? And this is the lesson about ethics that young people sometimes don't get. Ethics is not merely another bullshit that adults tell you as a part of their neverending attempts to manipulate you. Strong internal ethics is the thing that allows you to navigate through hard times, and then look in the mirror and feel good about yourself (and have a story where you are the good guy). Lack of ethics makes you weak, and tempts you to sacrifice parts of your soul for things that will feel worthless much sooner than you can imagine at the moment.