In broader terms the term abuser may refer to a tyrant or those actively participating in, or perpetuating sexual abuse and violent crimes like assualt against other people, as well as varying forms of oppressive or unethical behaviour. Abuse may cause isolation, or it might make other place inappropriate blame, yell, make threats, and engage in rude and objectionable behaviour like humilating attacks on intelligence, appearance or skill.
There will always be a group of users that persist in disobeying the rules, especially if they think it's harmless. Abuse can occur because of a lack of rules and law enforcement or it may arise from possessiveness and jealousy. Moderation exists as a tool to limit the effects and remove the presence of an abuser, but even moderation can be abused.