In most jurisdictions, defense of self or of others is an affirmative defense to criminal charges for an act of violence. It acts to provide complete justification.
Justification does not make a criminal use of force lawful; if the use of force is justified, it cannot be criminal at all: Dennis J. Baker, Glanville Williams Textbook of Criminal Law (London: 2012) at Chapter 21.The defense of justification (Penal Law art. 35) affirmatively permits the use of force under certain circumstances… The defense does not operate to excuse a criminal act, nor does it negate a particular element of a crime. Rather, by recognizing the use of force to be privileged under certain circumstances, it renders such conduct entirely lawful…In this regard, the current statutory defense reflects the common-law “right” of an individual to repeal a threat to life or limb… Defense of oneself or one’s relations, deemed a natural, inalienable right at common law, justified the use of force, making even homicide lawful.[2]
The defense of justification would fail, for example, if a defendant deliberately killed a petty thief who did not commit robbery and who did not appear to be a physical threat. However, the owner or lawful possessor of property has a privilege to use any degree of non-deadly force necessary to protect his possession or recover his property, regardless of no physical threat to his person.[citation needed]
‘Property’ is more than just the physical thing-the land, the bricks, the mortar-it is also the sum of all the rights and powers incident to ownership of the physical thing. [T]he right to use the physical thing to the exclusion of others is the most essential and beneficial. Without this right all other elements would be of little value.’[3]
The ownership and possession of property confer a certain right to defend that possession, [including] a defense of it which results in an assault and battery, and that which results in the destruction of the means used to invade and interfere with that possession.”[4]
In Cross v. State, 370 P.2d 371 (Wyo 1962) the Court found that the Due Process of Law clause in the state constitution guaranteed “the inherent and inalienable right to protect property.”
However, when an assailant ceases to be a threat (e.g. by being tackled and restrained, surrendering, or fleeing), the defense of justification will fail if the defending party presses on to attack or to punish beyond imposing physical restraint. A somewhat less obvious application of this rule is that admitting the use of deadly force in an attempt to disable rather than kill the assailant can be construed as evidence that the defendant wasn’t yet in enough danger to justify lethal force in the first place.
http://en.wikipedia.org/wiki/Right_of_self-defense