If you mistreat someone, you harm them in some way. If you don't feed your cat for three days and refuse to clean its litter box, you are definitely mistreating it.
People mistreat each other in all sorts of ways, from the emotional (by belittling, ignoring, or mocking them) to the physical (by hitting or otherwise physically harming them). If you see a neighbor mistreat his dog, you might call the police. If a teacher were to mistreat her students, she could lose her job. This verb adds the "bad or wrong" prefix mis- to treat, from its Latin root tractare, "manage or handle."