He who fights with monsters should look to it that he himself does not become a monster. And when you gaze long into an abyss, the abyss gazes also into you. -Friedrich Nietzsche
We don't have to look very far to encounter the reality of evil in our existence. I sometimes try to look back to when I first developed a concept of evil and realized that horrific things exist in the world. Go figure, I cannot. Evil is one of those things you just kind of know about, a timeless concept like goodness, life, or death. For years I've had a fascination with evil and the origin of all the horrible things individual humans and this race as a whole is capable of. Of course being an international relations major, in addition to my interest in history, has opened up a plethora of horrors about the limits of human morality and sanity.
But what is evil: an absolute, universal concept or something subjective? Where does it come from: ourselves or a higher power or force? Is evil a preventable thing, or a tragic and intrinsic part of existence? Most contentiously, is evil truly 'bad' or does it actually serve some sort of positive aim, such as defining what is good or keeping us united? Unsurprisingly there are range of philosophical and theological theories regarding the nature and existence of evil.
Socrates (or was it Plato, or both, I forget) believed evil to be merely ignorance, with good being merely that which all humans desire. Nietzsche assessed that whatever is branded as evil is only done so because it can't be obtained, so rather than deal with not having their desire, man copes by rejecting it as evil. Spinoza viewed evil as being dependent upon one's own personal inclinations, popularizing the idea that evil is as subjective as any other ethical or moral concept.
Perhaps my favorite interpretation comes from Carl Jung, who famously - or infamously - defined evil as "the dark side of God," and even interpreted the story of Jesus as being God facing his own darkside. Of course this was applied to humans as well, in which evil is committed by those who cannot face their own repressed weaknesses. Perhaps we can interpret the committing of evil acts as some twisted form of compensation. The concept of the Shadow in Psychology is often interpreted as suggesting that evil as an intrinsic part of all humans, repressed within themselves. Could it be that we all have the potential for evil, and that deep down we all have temptations to rape, murder, steal, and such? What does that say about our own nations, much less the nature of evil, which would thus be natural as opposed to the aberration it is often considered to be.
Plato observed that there are relatively few ways to do good, but many ways to do evil. Thus promoting goodness and morality could possibly not be enough evil; evil must be prevented or stopped because good alone good only do so much.
Then we have the religious arguments. Faiths like Christianity and Zoroastrianism (the latter having inspired the former), treat good and evil as two separate and conflicting concepts represented by two ultimate personifications; with humans and this Earth being fought over by the factions. Christianity takes it a step further with the concept of original sin, which seems to suggest that humans are naturally evil. Interestingly the prophet Isiah projected all things in the world to be originated from God, including evil, dispelling the seemingly classic idea of evil versus good, which is God. This supports the philosophical understanding of evil as a tool.
Judaism interprets evil in several ways: as the result of forsaking God or as a way of testing humans, who have a choice in the matter (Satan is even seeing as somewhat working with God to test humans).
In Islam, God is interpreted as being a separate being, and thus evil as a separate concept beyond his machinations: rather it originates in man as well. Buddhism's take is similar, best explained in the Buddha's message that "By oneself, indeed, is evil done; by oneself is one defiled. By oneself is evil left undone; by oneself, indeed, is one purified. Purity and impurity depend on oneself. No one purifies another." Hinduism's take often seems similar, though is more complex, holding that evil is a result of ignorance of one's inner spirituality, existing due to lack of wisdom. Islam and Buddhism have made similar notions as well.
And for the record most of these interpretations are just that, interpretations; so take what I have learned or gleaned from a faith with a grain of salt. Don't hold it against me if I have 'misinterpreted' any religion.