If you maltreat someone, you treat them very badly. It's best not to maltreat your little sister — she may grow up to be bigger, stronger, and smarter than you. Also, it's just not very nice.
If you're cruel to someone on purpose, you maltreat them. You can also say abuse or mistreat. If a big company maltreats its workers, they may rebel by going on strike or joining a lawsuit. This verb combines the prefix mal-, "badly, poorly, or wrong," and the word treat, from the Latin root tractare, "manage or handle."