I'm not sure what you are asking here, but a murder is defined legally in most states as [i]killing with willful intent in the absence of defense[/i]. Not every killing is a murder.
For instance, if a person were to accidentally run over and kill a man, then this does not constitute "murder." It is simply an accident that resulted in death. In most states, a person would not even face criminal charges if they killed a person (even a child) who simply ran into traffic.
If a person was upset at another person and, out of rage, ran that person over, it does not automatically constitute the legal definition of murder -- even if the victim dies. A prosecutor must prove willful intent -- that the accused knew or wanted the victim to die. In Texas a couple of years ago, a woman ran over her husband over and over again when she found out that he was cheating on her. The jury poll revealed that this crime constituted capital murder only because she ran over him over and over again. Had she hit him once, the jury probably would have acquitted her from the most serious charges even had the victim died.
A person who protects themself or another person is not guilty of murder if they practice proper restraint. This is not just limited to self-defense. For instance, Texas recently passed the "Castle Law" -- which protects property owners from charges if they kill an intruder and the resident fears for life or limb. Legally, this amounts to "justifiable homicide."
Of course, a soldier who kills during time of war is not necessarily guilty of murder. They are not killing to simply and intentially rob a person of their life. Rather, they are disabling a person in order to preserve life.
Murder can happen by action or omission. For instance, if a person neglects to help someone or act on a third party's behalf, they can be found guilty of murder.
There is quite a bit of debate about whether or not killing is ever justified according to the Scriptures. The Old Testament is filled with examples of killing as a result of the command of God. The New Testament urges believers to "turn the other cheek" in regards to themselves, but it is not entirely clear about the defense of others (such as defending a wife, a child, or a country).
Anyway, I hope this helps!