To victimize is to make someone into a victim by harming or taking advantage of them in some way. A school bully might be known to victimize other kids on the playground.
A victim is someone who has had something bad done to them, like the victim of a murder or a burglary. To victimize is to put someone in that position. A serial killer victimizes the people they kill. A swindler victimizes the people they cheat out of money. Abusive parents victimize their children. A football team that cheats victimizes the other team. When you hear about victimizing, you know someone has been hurt.