This is a common mistake made by native speakers because this word has been politically redefined.
If an accident occurs, the person is a casualty because there is no one to blame for the accident.
In order to have a victim, there must be an assailant, a victim of a crime or of negligence.
If there is an accident in a company. The casualty may want to claim that the company was negligent and caused the accident. His attorney will refer to him as a victim. If the casualty can proved that the company was negligent, then he becomes a casualty.