It might be asked: "If justice applies to people who consent, why would it not be seen as injustice to enforce rule over those that do not consent? Or does justice not apply to compact/consent at all?"
I don't think enforcing a rule over those who do not consent would be a matter of justice, would it? Probably that would be called "force" or something like that, but if the people involved have never consented to the agreement, then forcing them to do something is not a matter of "justice" at all, would it be? The point being that unless there has first been consent to an agreement, "justice" is not something that has any applicaion, because unless there has first been an agreement to bring into relevance a discussion of justice, then "justice" does not apply, as it does no exist in the air.
Also: "So if injustice is a term applied to those who withdrew consent, the individual who never consented to a compact could claim fault with an authoritarian system, but cannot claim it as "injustice" but rather must define the specific acts, like extortion or aggression etc.?"
Yes that is the way that I see Epicurus' formulaton applying. By no means am I suggesting that the person who never consented should not act to revolt against the authoritarian system, I am suggesting that the person who revolts should not view his actions in the prism of "justice" because that is a limited absract construction. The true and real analysis of all that matters in life is considered in terms of pleasure, pain, an