ANSWER: Self-defense is the act of protecting oneself or others when the threat of bodily harm exists. Interestingly, this phrase was first used in reference to fencing (1728 A.D.), then to boxing a century later (Online Etymology Dictionary located at dictionary.com).
In the Bible, killing in self-defense is somewhat related to our modern definition of manslaughter. Generally speaking, the killing of another person without malice (intent) or premeditation (the killing was not planned) would be considered (in the United States legal system, and no doubt others) a form of manslaughter. What is called "involuntary manslaughter" occurs when a person is killed due to an accident by another (e.g. your car brakes fail and you accidently hit and kill someone). A charge of voluntary manslaughter happens in the "heat of passion" such as when someone fights and kills another person with a strong blow.
Many people apply the sixth commandment in the Bible to the question of self-defense. The difficulty in answering this question is due in part to different translations. The King James Version translation of this commandment says we are not to kill (Exodus 20:13, also see Deuteronomy 5:17). This translation of the commandment would mean that God does not approve of the use of deadly force to defend oneself regardless of the circumstances. The NKJV translation, however, as well as most modern translations (such as the NIV, NASB, TEV and others), translate verse 13 of Exodus 20 as "You shall not murder." Jesus confirmed the latter translation of the commandment when he said we are not to murder (Matthew 19:18).
Our natural reaction to those who INITIATE violence against us and wish to do us grave bodily harm is to protect ourselves and those we love. Jesus said if a homeowner knew a thief was coming to his house he would not allow his home to be broken into (Matthew 24:42 - 44). This example shows that God gives us the right to protect our possessions and to do what is needed to defend ourselves (even though it says nothing about killing someone). In this regard Ecclesiastes 3 states the following.
1. To everything there is a season, and a time for every purpose under the heavens: 2. A time to be born, and a time to die; a time to plant, and a time to pluck up what is planted; 3. A time to kill, and a time to heal . . . (Ecclesiastes 3:1, 3, HBFV throughout)
The Jamieson, Fausset, and Brown's Bible Commentary on Ecclesiastes 3 says verses 1 and 3 states WHEN this killing is permitted " . . . judicially, criminals; or in wars of self-defense; not in malice. Out of this time and order, killing is murder."
In Numbers 35 God gave instructions regarding six cities where a person might take refuge if he or she has killed another person. These cities of refuge were created by God to prevent someone who killed by accident from being killed themselves before they could stand a fair trial (Numbers 35:9 - 13, 15). Numbers 35 gives some examples of death by accident and how God wants the situation handled.
22. But if he thrusts him suddenly without hatred, or has thrown something on him without lying in wait, 23. Or with any stone with which a man may die if he does not see, and throws it upon him so that he dies, and was not his enemy, neither sought his harm, 24. Then the congregation shall judge between the one who kills and the revenger of blood according to these judgments (Numbers 35:22 - 24)
The JFB Commentary confirms the purpose of Israel's cities of refuge as it relates to defending yourself. It states that when a person killed someone due to a sudden provocation or out of passion (unknowingly killing someone), the cities of refuge (Bezer, Golan, Hebron, Kedesh, Ramoth-Gilead and Shechem) offered full protection.
It is interesting to note that, even though the cities of refuge no longer existed, churches were considered places of refuge in the Middle Ages. Our modern version of a place of refuge is putting someone who kills in jail so that they can live to stand trial. The above principles should give you plenty to think and pray about regarding self-defense and the Bible.