I figured I would begin a new thread to address this issue.
Many military ethicists, moralists, and theorists argue that the justification for killing another comes from a liability; basically, the person you are killing has somehow lost or forfeited their right to life.
Having been a soldier who felt that I never explicitly, nor even implicitly, gave up my right to life, I began to research and investigate into this idea and the entire theory of natural human rights.
A natural human right is basically that which you are entitled to simply by virtue of being a human: life definitely counts, liberty probably counts, property is a bit more problematic, education and health care do not in any way fall under the umbrella of natural human rights. Further, there are certain qualifications of natural human rights, the primary qualification being alienation (e.g., We hold these truths to be self-evident, that all Men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life.) Alienation has two criterion, the first being the ability to transfer the entity in which you hold the right; nobody can transfer their life to anyone else, hence it is inalienable (the second criterion is that such a transfer, if it were possible, would have to produce greater value for the individual partaking in the transfer; Hobbes expresses quite clearly that nothing can be more valuable for any individual than their own life).
From that basis, I then investigated the notion of "forfeit" and concluded that forfeiture is simply a species of transfer (quick breakdown: "giving something up" is "giving"; "giving" is transferring; moreover, nothing reverts to nothing, i.e. one cannot give something to nothing). Thus, since one cannot transfer their right to life, one cannot forfeit their right to life. So, the justification for killing someone cannot be in the notion that said person has no right to life.
There must be a manner in which one can be justified in killing someone in order to preserve their own life, though. Working off of the notion and criteria of the duty to rescue (i.e., B is in peril; A is in a situation in which A can be reasonably successful in rescuing B; A can perform this rescue at a reasonable cost (which I simply place between 0 and 1); A has a duty to perform the rescue), I constructed a formula in which one can kill in order to preserve their own life:
A is morally justified in overriding Bs right to life (i.e. killing B) if, and only if: (1) the threat to As right to life is imminent; (2) overriding Bs right to life defends and preserves As right to life; and, (3) there is no other viable method in which A can defend and/or preserve his life.
This formula trades upon an innate valuing of one's own life above all others (which may or may not be accepted); so that A is justified in overriding B's right to life (solely in accordance with the above formula) because A's right to life is more valuable to A.
That simply gets one to a justification to defend and preserve their life, and only their life, with deadly force. Unfortunately, because the right to kill is entailed within the right to life (there is no liability/culpability mechanism at play), and one cannot transfer their right to life, then one cannot transfer their right to kill. In order to kill as a "3rd Party" one must first put themselves in harm's way, that is put themselves in a situation in which the only manner in which they can preserve their own life is to kill.
