In the OT, we find the law saying, "Thou shalt not kill." Although the term râtsach could mean both premeditated, unpremeditated, or accidental killing, its use in the Bible is almost always indicative of premeditated killing which we call murder. Warfare (or protecting your family for that matter) was treated as another thing entirely. Remember, the same God who told Israel not to murder also commanded them to go to war for the land that He was giving them. Not only that, but they were told to leave nothing alive of the people they were replacing.
But we live under the new covenant right? Some would say that the standard is now higher than under the old covenant, as though the old law has been replaced by a more comprehensive law that now judges motive along with action. The book of Hebrews is very plain on the matter. The old covenant has not been amended or revised, it has been abolished. Did you know that the law that said we should not wear clothing that was a blend of two fabrics (i.e. wool/cotton blend) and the law that said we should not eat pork are equally part of the law that says we should not commit murder. The ten commandments do not hold some kind of special status in the law that prevented their abolishment as well. The law is the law, all of it.
So that means we can kill right? What did Paul say in Romans? Since we are free from the law, shall we continue in sin that grace may abound? God forbid! How shall we who are dead to sin, free from sin, live any longer therein? What is sin? Well, whatsoever is not of faith is sin. All other falls under this category. Disobedience is sin. We are to walk in the Spirit and not fulfill the lust of the flesh. We do this by being obedient to the promptings of the Holy Spirit and to the written Word. If the Holy Spirit prompts us to go speak to a neighbor and we disobey, it is sin unto us. The new covenant is NOT the law plus a further addendum that raises the bar. The New covenant is absolute abolishment and nullification of the old covenant, including the ten commandments. Is the "standard" higher under the new covenant? The problem with this question is that is starts with a faulty premise, that being that we are still trying to be right with God by keeping a "standard" at all. We are to live by the spirit, not by a "standard" of godliness written down in rules, regulations, thou shalts, and thou shalt nots. We walk in the spirit, not in the flesh. Israel could not do that. Their entire relationship with God was bound up in keeping ordinances. Praise God I am free from ordinances.
Praise God that my sin was forgiven and dealt with 2000 years ago. Praise God that 1 John 1:9 applies to me as a believer. Praise God that if I forget to confess a sin, my spirit which has been perfected forever (Hebrews 10:14) does not become "un-born again". Praise God I live under grace.
God has given me a family and ordained that I be the head of that family. In that, I am called to serve my family, teach my family, lead my family, and protect my family. I made a vow on my wedding day to protect my wife. I personally would not think twice about pulling out my firearm and using it to protect my wife or children were a "meth crazed" intruder to come through my door trying to harm them. I am believing God actively that this scenario never happen. I see this as different entirely from my wife giving her life as a martyr from Christ, or myself for that matter.
Hope I have explained my opinion clearly. There is a lot of explanation involved that is a bit difficult to verbalize.