Mistreatment is the name of an action: treating a person or an animal badly, especially in a cruel way. It is a noun, not a verb — you mistreat someone, and that action is mistreatment. The word is heavier than “being unkind.” People use it for hitting, starving, locking someone up, or wearing a person down with fear and harsh words, often over time. News and law use it a lot: child mistreatment, mistreatment of prisoners, mistreatment of animals. Though actually, people stretch it. A worker might call a rude boss “mistreatment” even when nothing illegal happened, and listeners sometimes argue about where “strict” ends and mistreatment begins. The word does not, by itself, tell you how bad it was; the rest of the sentence has to do that.