Mistreatment is being cruel or thoughtless toward a person, animal, or even an object. Your little brother's habit of picking the cat up by his tail is an example of mistreatment.
Mistreatment of the family car might include driving it on the beach, while mistreatment of one's children could be as serious as abuse. If you don't treat someone well, it can be called mistreatment. The word itself comes from the Old English prefix mis-, "bad or wrong," and treat, "to handle or deal with."