Conscience

The 1988 Webster's dictionary defines conscience in the modern sense as
the faculty power, or inward principle which decides as to the character of one's own actions, purposes, and affections, warning against and condemning that which is wrong, and approving and prompting to that which is right;
the moral faculty passing judgment on one's self;
the moral sense.

Conscience as society-forming instincts
The human animal has a set of instincts and drives which enable us to form societies: groups of humans without these drives, or in whom they are insufficiently strong, cannot form cohesive societies and do not reproduce their kind as successfully as those that do. They either cannot survive in nature, or are defeated in conflict with other, more cohesive groups.
Behavior destructive to a person's society (either to its structures, or to the persons it comprises) is bad or "evil". Evil or wrong acts provoke either fear or disgust/contempt. Thus, a madman who threatens us with a chainsaw and one whose sexual practices we ourselves find revolting might both be labeled "bad". Indeed, one does not necessarily need to do anything to be "bad" - a natural coward may provoke contempt, and thereby be a bad person (ie: a coward), even without actually having any occasion to flee from the enemy. And the identification of badness can be quite subtle and involve reasoning. For instance: a sheriff that shoots a gunman is not thereby bad because he is not a threat to an average member of society (as the gunman is), and hence does not provoke fear. Yet gangs of criminals can perceive law enforcement officers as bad people.
Conscience is what we call those drives that prompt us to avoid provoking fear or contempt in others. We experience the operation of conscience as guilt and shame. We feel guilt when we perceive that others might rightly fear us, and shame when we perceive that others might rightly find us disgusting or contemptible. To avoid these negative and unpleasant feelings, we modify our behavior: thus "conscience" prompts us to behave "rightly".

A requirement of conscience, then, is the capacity to see ourselves from the point of view of another person. Persons unable to do this (those suffering from psychopath, sociopathy, narcissism) therefore often act in ways which are "evil".
Another requirement is that we see ourselves and some "other" as being in a social relationship. Persons trying to resolve conflict between groups try (and sometimes succeed) to create a feeling that a social relationship exists, that the groups in conflict all belong to some larger encompassing group. Thus, nationalism is invoked to quell tribal conflict, and the notion of a brotherhood of man is invoked to quell national conflicts. There are even appeals to relationships between ourselves and the animals in society (pets, working animals, even animals grown for food), or between ourselves and nature as a whole. The goal is that once people perceive a social relationship, their conscience will begin to operate with respect to that former "other", and they will change their actions.