I was bullied at grade school all the time, I was a poor, fat kid and my parents had just divorced, I was happy and optimistic, but very vulnerable. I won't go too much into the details of this, because this post is already too long. But as a result and the point of this post is that I became a bully myself.
I didn't have friends there and as a result I hanged around with the kids on my neighborhood. They were all younger than me and quite a lot of immigrants and others with not that much of a "social standing" and I could get an easy hold over them just by the power of seniority. Then, when I went to a middle school and the scenery changed, I had become an alpha male in that sense that I knew how to rule over people with fear and verbal abuse. I didn't do it consciously, it just had became a part of how I act.
Bullying is about abusing other people's weakness'. I had became a master of it. I never actually did anything myself, I just talked, other kids did stuff for me. Physical abuse like "Hey I know, let's fill your backpack with all of our books and hit that guy in the face with it!". Stealing shit from school like computer equipment, cigarettes and candy from grocery stores. There were even plans to rape certain girls. Perhaps the only time I thought I maybe had gone too far was when a "friend" of mine bought a gun to school.
The way the school tried to control me was to talk to the people around me. My closest acquaintance was this huge football guy and the principal frequently took him into talks and tried to get him to stand up against me, he always came back to me to tell it and my reaction was pretty much "Haha what would you do you fucking fag?". In reality, if even one person had said anything or done anything to me, I would've completely crumbled down, because inside I was still that bullied little fat kid trying to fit in.
What I'm most ashamed is the bullying I inflicted on one kid that was born with his legs different height and as a result, he didn't walk very good. Easy target.
Also, my own little sister. She was the first one in line always. We fought so much that she eventually had to move away to live with my father.
Later on in life, I found out that the kid who began the cycle of hatred, my original bully, had a very abusive big brother.
I've gone through clinical depression, severe anxiety and drug abuse at my adult life to truly find out who I am and I feel that a lot of it was residual from my childhood, as it usually is.
I'm not trying to justify anything, but if you are in that period in your life where you don't really know how life works, there's nobody to show you and the only experience that you have is dog eat dog, then that's what you become.