I am very sorry that you were victimized and I hope that you read my words as they're intended: as strong suggestions from personal experience.
Bullies and abusers (this includes molesters, rapists, and many corporate executives) target people who appear "weak," vulnerable, or different from themselves. With teenaged bullying, it's pretty much peer pressure/acceptance and herd mentality. With adults, it's something that cannot be accurately understood because these types of people are not going to "confess" anything. They bully and commit atrocities simply because they can. They get some kind of satisfaction and/or sense of power/control from the damage that they inflict upon others. And, that's it.
You will never be able to "understand" what drives these people because you do not live in the same Universe that they do. Understanding them, their motives, or what they believe that they accomplish from their behaviors wouldn't alter the types of people that they are, either. The only thing that we can do is to know that they exist, accept that they are not going to change, and learn to recognize their predatory behaviors and protect ourselves from them by building strong, healthy boundaries.
It may be a very good decision to engage in some strong counseling therapist with a specialist that deals specifically in trauma and abuse. These professionals will help us to process our experiences in a positive, healing manner, and help us to identify our own precious vulnerabilities, strengths, and qualities so that we can build boundaries to protect them.
Best wishes to you.