There is no duty to treat in Maine. In order you use deadly force the person in danger must believe it is "necessary to prevent the infliction of bodily injury." It is also legal to prevent kidnapping, or robbery. Deadly force may be used outside the home on the owners property only to prevent arson or criminal trespassing.
Castle Laws are laws that address the use of force when defending one's self inside their home, or on their property. Some states expand this to vehicles, and the person's place of work. Castle Laws generally include: the places where this law applies, the requirements fro use of deadly force, if there is a duty to retreat, the amount of force that maybe used in defending one's self or others. These vary widely from state to state.