bad actor

A person, animal, or element that is unreliable, unruly, and prone to troublesome or quarrelsome behavior. The class was full of bad actors, always fighting and causing trouble. The neighbors have quite a bad actor for a dog—it's constantly trying to fight with our dogs. A number of bad actors in the financial sector have been exploiting loopholes in the rules, making it much harder for those of us who do business legitimately.