Can you tell me if bullies ever realize bullying may hurt someone?

Yes: I suspect that all bullies realize that they are hurting someone. The problem is, that most bullies simply don't care. Among adults many of them have personality disorders such as narcissistic personality disorder, borderline personality disorder, antisocial personality disorder, and sometimes what is referred to as malignant narcissism. This is where i think the dsm fails us in not having a sadistic personality disorder category.The german word for sadism, schadenfreude, literally means to get joy out of inflicting harm on someone else whether it's psychological or physical. There are certainly many people in the world who fall under these categories.

Inferiority: Believe it or not, bullies tend to have inferiority complexes, often from having been bullied themselves. and yes, it can continue into adult life. Tell someone you know and trust about the bullying. Keeping it inside does no good.