Disclaimer: I'm not infallible, nor do I claim to be. I'm always seeking to grow, adapt and refine my position. I'm confident in my position and will defend it, but I'm willing to modify or change my position when presented with new information. If anything is unclear, or needs elaboration, just ask! If you're gonna be a jerk, I will not respond. :)
In order to have a clear and productive conversation, definitions must be established.
The following is the definition of the Zero Aggression Principle (AKA Non-Aggression Principle) as I understand and apply it:
The ZAP is a moral and ethical principle which holds that the initiation of aggression, coercion or the threat thereof, regardless if the action has a positive, negative or neutral effect, violates the property rights of the individual, and is therefore immoral and unethical.
The scope of this definition is meant to be broad and well defined in order to resolve disputes that may arise over issues of ownership and private property.
It is important to emphasize that it is the INITIATION of aggression that is considered the unethical/immoral action. You as a self owner, hold a property right over your body, and are allowed to defend yourself with deadly forced if necessary. With the right of property comes the responsibility to take care of and maintain that property (Not that anyone can use force to compel to do so, of course).
Another important point is regardless of the intent of the aggressor, be it to benefit, harm or otherwise effect the victim, the very act of aggressing upon someone is ground for them to respond in self-defense. If I were to grab you by the neck and shake you, while stuffing $100 bills into your pockets, you would be justified in using force to repel or kill me.
The ZAP is not a contract, as some critics have a tenancy to assert. That being said, the ZAP can, and has been used in contract law, but the principle itself is not a binding agreement.
For example, I personally abide by the ZAP, and will not initiate aggression against another. However that doesn't stop anyone from aggressing against me. I understand this, and will defend myself and my property if the need arises. Just because I'm a peaceful person doesn't mean I'm a pacifist. :)
An example of contract use would be if a neighbor and I voluntarily agreed to not trespass upon each other's land. It is again present when I enter a store. I enter under the conditions that I do not steal or destroy any property of the store owner.
One last point to wrap this up.
When I argue for the right of an individual to, for example, smoke dope, I am not, I REPEAT: AM NOT, advocating that you, or anyone else, smoke dope.
I make the argument based on an ethical or legal basis: "What am I allowed to do here and now that won't encroach upon another?"
Advocating the right to do something is not condoning that act, it is asserting the property right of the individual.
Next up, I will try and deliver a basic and concise theory of property from a Libertarian perspective.