You can use the noun transgressor for anyone who violates a rule or oversteps a boundary. You are being a transgressor, for example, when you bring your barking dog into the library.
A car thief is one kind of transgressor, a person who is actually breaking the law. Another kind of transgressor is someone who betrays an agreement or an unstated rule of behavior, like not wearing full clown makeup to a non-circus-related job interview. The word transgressionem is at the root of transgressor, and it means "a transgression of the law" in Late Latin or "a going over" in classical Latin.