Answer and Explanation:
First of all, it is necessary to talk to Marty and ask the reasons that are causing him to act this way and how he feels about it. That's because Marty may be facing some bad situation and that has been the way for him to act that way with other children. After listening to him, the adult should make sure that the triggers are eliminated from Marty's life, probably looking for a psychologist who can help him with this, but it is important that this adult talks to Marty, explaining the reasons why his behavior is wrong and cannot be tolerated.
Then, this adult must show Marty how to avoid this type of behavior and if this continues, Marty will be punished. This punishment will remove from Marty what he most likes to do, such as playing video games, internet access, weekend trips, or something else that Marty likes a lot. Whenever punishment is imposed, Marty should receive an explanation of why he is being punished and how he should behave.