The sense of morality we in the civilized society share today derives its origin from the necessary condition of the establishment of civilization. In order to maintain harmony and to minimize conflict between distinct members of society, people naturally agreed to certain “ground rules” (explicitly or implicitly; voluntarily or by force), often called the “Social Contract” (or the agreement that every member of society must agree to upon entering society), that served to achieve that. The fundamental goal of those “ground rules” is to preserve individual freedom insofar as it does not interfere with that of others; but to limit it where it unfairly intrudes upon that of others.
The most basic rule established to maintain order in society is one against murder (in any form) (There are many others, but I believe my point is clear with the description of only murder). Before civilized society emerged from the stage of tribal warfare where the object of each (male) member of a tribe is to either obtain food (kill a mastodon) or to eliminate competition (destroy other tribes), humanity was stuck in an age of static existence–there was no progress. Only when people organized into large communities of interdependent members did they step into the path to growth and progress. But that level of organization was strictly prohibited by the primitive social structure of the pre-civilization age–there can be no community that grows by nurturing the growth each of its members until the habit of tribal (or inter-family) “competition” (of the Darwinian sort) ceased. In short, there can be no organization until humans are ready to organize.
Hence was developed the basic notion that one ought not to kill his neighbors–because progress will be completely stifled by the constant fear of disorder (being killed, robbed of our means of livelihood, raped, etc). In order to facilitate the transition from a primitive and predatory mode of human organization and behavior into a civil and progressive one capable of mass organization, people began to teach kids starting at an early age the basic morals that guide human behavior: the basic principle of non-intrusion (described in the opening paragraph).
As the centuries passed, those understandings of what we now call “morals” became firmly ingrained in the minds of humans and consequently we began to see them as the “right” way–but that only happened because it was conducive to the habit of organization (without ground rules, there would be only chaos). But right in what sense, nobody can explain without simply appealing to the sense of empathy: It “feels” right but there’s no “proof from principles”. Right in a universal way, as much as Maxwell’s laws are? Nobody makes an assertion that bold, but they also do not specify. Right in the sense that it promotes social welfare and human development? Perhaps–that’s the cause of its origin, and likely the cause of its continued existence. But is it right in any non-prudential manner?
I cannot think of any. Hence, it is not “right” because it “ought” to be right–that would be an extremely naive and circular argument; but “right” because it helps human society. Hence, everyone’s “right to life” is desirable insofar as it promotes social welfare–the greatest good for the greatest number of people. If a particular person’s existence is threatening or detrimental to the livelihood of his neighbors, there is no universal law that forces us to conclude that his life is as precious as anyone else’s, and hence that the “right” to life (apologies for the multiple use of the word “right”) is not universal, but only the consequence of the fact that the life may be “good” in some sense.
As we can see, the notion of the right to life originates from necessity from when humans organized into complex communities and is only a notion designed to promote human welfare because it could do so, not a universal law that supersedes all other prudential considerations. It was fabricated almost arbitrarily by a common-sensical social-engineer (or the ruling class in general) who understood that it is necessary. It doesn’t mean that the understanding is any less important, but that it should not have its universal level of formality that many assign to it.
Therefore, nobody has the “right” to life in the way described by the Enlightenment philosophers, but only insofar as his living makes his condition or the condition of others better off. Hence, the convicted criminals of today do not have the right to life that most of us do–because they have demonstrated that their existence is harmful to the well-being of others; and that those who will be born into destitution are better not born at all.