One detail that seems to trip up many people when the Zero Aggression Principle is discussed is when it's OK to act in self defense before a serious, or even deadly, blow has been struck against you. I think the only time it is OK is when there is a "credible threat" of harm.
To me a credible threat is when, to the best of your knowledge, the threatener has the intention to follow through with the threat and the means to actually carry it out.
A braggart who just wants to look big in front of his friends, but who is too weak to actually do you any damage and who is unarmed (listen up, goverthugs in the Pentagon) is not a credible threat no matter what he says, and an armed person who has no intention of attacking you, even if they are justified, (listen up, LEOs) is also not a credible threat.
Of course, if you attack first anyway, he has no obligation to take it without fighting back, and once you attack, no matter who you are or what you think your "job" is, YOU are a bad guy. Even if you "win". If that is OK with you, then who am I to tell you to get a conscience? You do what you think you must and live with the consequences, all of them, like a self-responsible person.