Most of you here are well versed in the laws governing self-defense, but you may have the opportunity to inform others who are less knowledgeable. As an instructor for many years, I have encountered many misconceptions about the legal justifications for the use of force. A number of my students would have wound up in jail had they followed the word of mouth and urban legend they had heard. My God, if you are going to carry a gun, or you are going to use one for home defense, know the law. I mean print out your state statutes on the subject and study the language, and even better attend a class from a knowledgeable instructor. I say knowledgeable because there are a lot of well-meaning folks with instructor certificates who know not of what they speak.
Laws vary widely from state to state. In general, the fear of imminent death or great bodily harm to yourself or others is legal justification for the use of deadly force, wherever you are standing and regardless of weapons involved. Every situation is different, but justification in almost all use of force cases comes down to the reasonable man standard. That is, within the bounds of the law, what would a reasonable man believe is reasonable and necessary under the circumstances. Different states have different wording in the statutes, but this is pretty much a universal concept in the U.S. If you use deadly force, you may need to show evidence that you were in such fear.
Another justification for deadly force can be to stop the imminent commission of a forcible felony, but there is a wide disparity among states on this issue, you must know the law where you live.
Stand Your Ground is another concept, which simply says you do not have a duty to retreat from a place where you legally are, before using deadly force. The media has overused and misused the term so much that there is confusion about what it means. Not all states have a "stand your ground" provision in their statutes, and you need to be aware of what the statute clearly states.
The Castle Doctrine concept is often misunderstood to mean something that it does not. It does NOT mean that you have carte blanche to shoot someone who breaks into your dwelling or occupied vehicle. In some states deadly force is justified if someone is committing a felony against you in your home but there is a lot of room for interpretation. A more meaningful justification is that you are PRESUMED to be in fear of death or great bodily harm if someone forcibly enters your home or occupied vehicle, where you may not have that presumption if you are not at home or in your occupied vehicle. The difference seems subtle but here are two statements that are illustrative.
1. "The SOB made me mad when he broke into my house"
2. "The SOB put me in fear of death or great bodily harm when he broke into my door".
Depending upon which state you are in, statement #1 could cause you problems. Again, you must know the law where you live.
This post is written in general terms and is not necessarily definitive to your state. I am not an attorney. I can get really definitive about the law in Florida but that is my area of expertise. Did I mention read the statutes where you live?