"Victim" and "casualty" are very often not synonyms. A person can be a victim of bullying or identity theft or sexual harassment without being a casualty, a word which implies physical injury or death, typically in a combat or terrorist situation involving many people.
I speak English every day and I put casualty as it seemed the most likely choice of words for a scene of discovering, for instance, a 'victim' of a disaster. I might also say that If I were introducing someone who had been a casualty I may introduce them as a victim, for instance:- My friend Peter here was a victim of the avalanche. These are my choices. Needless to say I chose casualty and was marked wrong. Most of my errors it would seem are in English and I'm not attempting to learn English.