ok but suppose the two people involved in that murder hypothetical were on a desert island totally isolated from all organized communities or other people entirely? Would self defense then still be a matter of "justice"? Now clearly it is desirable / proper under Epicurean texts, but the question is must Epicurean justice be a matter of "society"?
Maybe that last part is the key point. Is Epicurus talking about justice using his own terminology? As with gods, how much of the outside terminology is he accepting?