There is a characteristic parameter that I would call 'Moral Strength', in people. The space over which this parameter is defined can be roughly partitioned into values corresponding to people who have a conscience, and those who lack in that (I also realise that I sound like a technical text book, but I can't help it as they are the only prose to which I've been exposed; maybe I should diversify my readings..? anyway).
Conscience is the ability to analyse one's actions and relate them to one's own morality, in an objective way, removed from emotional interferrence. The exclusion of interferrence by emotion does not mean that emotion is ignored, rather itself percieved objectively.
Wikipedia proposes a description of conscience in here. This seems to roughly agree with my proposed description of it.
A person with a sufficiently high level of moral strength has a disadvantage in life, in that he has some level of restriction on his behaviour. One with weak moral strength enjoys a limitless range of candidate behaviour. He is capable of anything; a second person is not able to predict, with confidence, how such a person might act or react.During childhood, a person develops a theory of morality for himself by watching the interactions and effects of those interactions in the world outside of himself. The knowledge that a person gains through this observation is stored as a repertoire for behavioural reference for his future. The manner in which a person makes use of this stored knowledge is a function of the level of moral strength that the person has.
A person with a high level of moral strength behaves as he does in a situation that is presented to him because he searches this stored knowledge for a way of reaction that results in creating a consistent and desirable environment. One with a low level of moral strength searches for a way of reaction that results in an increased level of pleasure and utility for himself. Therein lies the core difference between those to whom we would ascribe a conscience, and those to whom not one.
This, however, is not all or nothing. There is indeed a continuum between the two types with different proportions of concern for environmental consistency and concern for one's own pleasure. Where one lies within this continuum is a function of the human parameter called moral strength.
The question remains: Why would one place environmental consistency above one's own desires and goals? We are all independent agents, are we not? The answer to this question is rooted in two areas: (1) The concept of reality, and (2) Evolution, though both are intertwined.
The societal advantage of having a conscience is obvious. A society can be successful when people behave in a consistent and mutually acceptable manner. That is why we have developed systems of Law. It may be so that during the evolution of humans, we developed a faculty that induces some level of altruism in us. This would serve as a sort of distributed legal system. Because the notion of altruism is in direct conflict with most of the other in-built motivations that humans carry, the strength of this faculty remained inconsistent, and we developed Law, a centralised means of accomplishing the same goal. And so I propose this as a root, in evolution, for moral strength.
What do we mean by Reality? The only reality of which we can be sure is that we have no conception of reality aside from that through our sensual faculties (touch, vision, smell, taste), and we know that the sensual faculties of different people are different, and this creates a different reality for each. As an example, consider sharks, who have a sixth sense for sensing electromagnetic fields. Before humans learned of the existance of such a thing, EM fields were not a part of our reality. Furthermore, our senses can be modulated by our brain: we get used to noises and smells, and we don't hear or smell them anymore; if we wear eyeglasses that make everthing look upside down, after a while we start seeing right-side-up with the eyeglasses on. These things concern physical reality, and there is another dimension of reality in the world: This is the abstract reality of emotion, righteousness, fairness, balance. We humans do have a sixth sense: The sense of balance in the environment. This sense is manifest in our moral strength.
A high level of moral strength allows us to perceive an additional dimension of our reality, in the way that sharks can perceive EM fields, we can perceive the effects of actions on the subjects and objects of those actions. The doers and the ones to whom is done. We can evaluate the balance in this and apprais the state of this dimension of reality.
A very basic level of balance is guaranteed by our systems of Law, but law does not delve very deep into this reality of emotions and balance, righteousness and fairness. And the framework of morality does not provide a balance between those with contrasting levels of moral strength. The delicacy and sensitivity of this situation is of paramount importance to him who is living a human life. Balance between two people can only be maintained if the levels of moral strength in the two are similar.
The only apparent advantage of being able to perceive this dimension of reality is that it enables one to understand and be aware of a collective reality, and to understand oneself within this. Though the disadvantages of this ability are many, it may be so that the satisfaction gained from this understanding provides the balance that is seemingly lacking. But this is arguable, and I say this only on faith in human evolution and reality.
If we do not hold enough moral strength, we become blind to this dimension. In the same way that the brain modulates the physical senses, it can modulate our perception of this reality of balance, so that we perceive it in accord with the other self-centered motivations we carry. In the absense of sufficient moral strength, we can make ourselves believe that something we do that is cruel is actually justified, or even the right thing to do. The danger presented by such a situation to society is clear, but it presents an even greater danger to the self. It allows a human to live in a self dillusion that is likely to fall apart at any time and result in tremendous suffering.