Non Aggression Principle

There are various statements of the non-aggression principle (abbreviated NAP, or ZAP, Zero-Aggression Principle) (Wikipedia). I like Rothbard's statement of it:

No one may threaten or commit violence ('aggress') against another man's person or property. Violence may be employed only against the man who commits such violence; that is, only defensively against the aggressive violence of another. In short, no violence may be employed against a nonaggressor. Here is the fundamental rule from which can be deduced the entire corpus of libertarian theory.
— Murray N. Rothbard, A Libertarian Theory of War
This statement is good for specifically calling out self-defense. Note that aggression is against person or property (since a person owns themselves, there's some redundancy there). Frequently phrases like "the initiation of force is wrong" (or "aggression" for force) are used; initiation makes it clear that self-defense is not considered wrong; self-defense also includes hiring agents to defend you (although they have no special rights).

It is not specifically spelled out in the NAP, but libertarian thought usually allows only for commensurate defensive response (killing someone for stealing a loaf of bread, except in extreme circumstances, would likely be considered initiation and not response). The idea is to use necessary force to stop a threat—not pull punches, but neither cruelty or continued force when the threat is neutralized are part of self-defense.

Also external to the NAP is the idea of justice after the fact. Rothbard's "infringe as you were infringed", Block's "two teeth for a tooth", and Kinsella's "applied estoppel" principle are all good sources here. Please see the sources for Recompense for Fear: Is Forced Russian Roulette Just?.

Note too that the NAP is a principle, not a group of armed thugs. You won't meet it walking down the street. It is not self-enforcing; it cannot be. It can be violated, just like, say, the principle of "pacifism" or the Golden Rule, and it may be that nothing bad will happen to the violators. It's a just and moral principle for association in a free society; nothing more, nothing less.

It also is probably not enough for most people. It is a minimum; most religions would not call a person "good" (or virtuous) if they merely refrained from harming people. Beyond it, help to the truly needy (the Bible mentions widows and orphans in particular) is a command of many religions and something many people, religious or not, would require of themselves. The NAP just says it is wrong to coerce it.

Also see:

The Non-aggression Principle - various interpretations.
The Non-Aggression Axiom of Libertarianism by Walter Block