Why do people abuse children you mean? They're a lot of reasons. They could have been abused as a child and don't know any different, they probably thought it was normal or they deserved. If a child knows they are abused, chances are they won't abuse, if they don't, odds are they will. Physical abuse they obviously know, verbal abuse isn't as recognizable or emotional abuse. The parent could also have a personality disorder like narcism, which means you love your image but not so much yourself, you're sensitive to criticism and you have no consideration for feelings (look it up). The parent could also be an alcoholic or drug addict. Or they are just sick in the head. There are a lot of reasons.
They could have a drinking problem or they could have anger problems or have some kind of mental illness (you could do a lot with the mental illness if you take the time to look up a few) Or it could be family problems maybe there parent was abusive to them as a child and they may think its okay to hurt the child because their parents did it to them. There's soooo many reasons, you could probably find like a bazillion reasons just by watching Law & Order.