The use of deadly force may be justified only to defend against force, or the threat of force of nearly equally severity, and is not justifiable unless the defendant reasonably believes that such force is necessary to protect himself/herself) against death or serious bodily harm. Serious bodily harm is an injury that creates substantial risk of death, causes serious permanent disfigurement, or causes a protracted loss or impairment of the function of any bodily member or organ.
One cannot respond with deadly force to a threat of, or even an actual, minor attack. For example, a slap or an imminent threat of being pushed would not ordinarily justify the use of deadly force to defend against such unlawful conduct.
When you must defend yourself and the attacks claims injury, most people worry about their possible liability. There are two vastly different grounds for liability: criminal liability (law) and civil liability (law). There are two types of law: criminal and civil. Criminal law delineates rules of behavior that, when violated, may lead to punishment by incarceration or fine, or both. Civil law states which actions may lead to personal liability. Since these are two entirely different types of law with separate courts and procedures, a person found not guilty of a crime in a criminal court may be found liable in a civil court. The results of a criminal court have no effect a civil court, and vice versa. In a criminal court, the government tries and punishes a person for a criminal action against society. In a civil court, a person may be found personally liable for an action that injures another party.