Let us leave aside, if we may, the metaphysical question of the nature of good and evil. Let us simply allow that people, by and large, have a sense of things that they perceive as good and things that they perceive as evil. Regardless of whether or not such a view of things is justified, I wish to suggest that the relationship that most envision between these concepts is flawed. Perhaps it is this flawed conceptualization of the relationship between good and evil that causes such consternation when considering the nature of these principles in the public sphere.
Allow me to demonstrate the nature of this abstract confusion. Imagine a line segment (the length is not important but it is probably best, for our purposes, to imagine a finite and comprehensible segment rather than anything too large). Imagine that this segment is bounded on each end by a point. On the one end we may call this point "good" and on the other end we may call this point "evil". Imagine then some intermediary point on this segment, between the extremes of good and evil, and imagine that this point is not fixed, but may travel along the segment until it reaches the aforereferenced extremes. Thus, if this point were to travel in the one direction (towards the "good" say) it is coming ever closer to that thing we have called "good" and ever further from that thing we have called "evil". Likewise, if the motion were to be reversed, this point would come ever closer to "evil" and recede ever further from "good". Got that?
I posit that this thought experiment represents a fundamental human intuition about the nature of good and evil. That is that good and evil are opposite extremes of one another. If we recognize something as evil, then so long as we flee in the opposite direction away from it we are invariably approaching something that is good, and vice versa. Perhaps this intuition explains something about the extremes that we often see in human behavior regarding our standards of judgment, especially as regards judgment of complex and important ideas. Take the idea of socialism, for instance. If a person has deemed socialism an evil, than the opposite extreme--no matter how extreme it may be--is rather to be deemed a good. Likewise in opposition, if socialism is deemed a good, than anything falling away in the opposite direction is evil, so if one is to be good one must do all one can to approach the end goal of total socialism. Granted, socialism itself is somewhat lacking in concreteness in that there is much disagreement concerning its meaning, so let us consider something rather more immediately understandable. Gun control, for instance, is a topic that garners strong intuitions concerning categories of good and evil. Thus, if one perceives gun control to be an evil, than to flee against any form of gun control is by definition good (and the further one flees the better!) and to approach it is to approach ever closer to an evil; and likewise vice versa.
Now one might quibble that these categories are presented in an unrealistically Manichean fashion, but leaving this objection aside, we can see that such an equal opposition of categories of good and evil presents us with a universe that is essentially dualistic. If we make this choice willfully and with understanding, than so be it. However, an oft overlooked principle of dualism is that the fundamental opposition of good and evil leaves us with little in the way of providing guidance of which opposing force is which, other than our own personal preference (the fact that some consider gun control a good and others an evil illustrates this point). That is, if we have two opposing and opposite forces, than what determines which force is good and which force is evil? If we answer that the good is determined by that which references apriori principles of goodness (such as life being obviously attributable to good as opposed to death which is attributable to evil) we really haven't answered our question, because we have only subjected these apriori judgments to another level of moral analysis which would itself suggest a new line segment with good and evil hanging on either end.
But this objection, while relevant to our current discussion, does not strike at the heart of my objection to this moral dualism, it merely speaks to yet another intuition which asks us to question whether our previous intuition (i.e. that good and evil are opposite and opposing forces) may itself be inadequate. That is, might our abstracted line segment (and the intuition it represents) not be a very good model for the existing moral universe? May I suggest an opposing image of the relationship between good and evil. Again let us imagine a line segment. But this time, rather than "good" and "evil" sitting as opposing poles on either end of the spectrum, rather "evil" represents a pole at either end of this line segment and "good" inhabits a fixed point lying exactly half-way between these extremes of "evil". Thus rather than presenting "good" and "evil" as opposing extremes, rather "good" represents a mean between two radically opposing visions of "evil". "Good" then cannot be effectively approached and "evil" avoided by picking a direction along this line and moving inexorably in either one direction or the other, for one is like to overshoot the mark. Rather, one must cleave to a narrow course, where to yield either to the one side of the other is to shrink away ever towards two extremes of darkness, depending upon where one happens to be in relation to the "good".
There are obvious objections that can be made of this model, but I wish to suggest that this model better represents the relationship between good and evil than does the dualistic model. I suggest this for two reasons, one because it better fits the sum totality of our moral intuitions and two because it allows us (without reference to an apriori appeal) to judge that good is better than evil (there are objections that might be made to this second point but I am not going to address them here). However, while this second model better fits the appearances and allows us a conceptual framework to hold good as better than evil, it distresses us in that it makes the approach to that which is good so much harder and so much more indeterminate than it is in the dualistic system. For in this model any project, carried to a certain extreme, will inevitably miss the mark and lead us inexorably into error and towards evil. This is a disturbing thought, yet it is a thought befitting a grown-up conceptualization of ethical metaphysics, which embraces at one and the same time both the existence of good and evil and the recognition that there are no easy methodological means to determine the appropriate course of action in any particular situation.
Ours is an age of sloth and ease and it is unlikely that many shall hue to this harder course. That is a true shame, for in our misunderstanding of good and evil much damage shall be done--in the name of both good and evil.