We live in a world of possibility, risk, and freedom. A world of freedom means a world of potential good and potential evil - light crashing into darkness, beauty marred by perversions.
The essential nature of evil is that of a black hole: merciless, malicious, destructive vacuity. Evil always has to do with nihilism and emptiness - the reversing of God's positive and procreative "Let there be..." to "Let there not be," or "Let there be nothing."
In The Silmarillion, J.R.R. Tolkien masterfully demonstrates these properties of evil as he describes the spider Ungoliant, an accomplice to Melkor:
She had disowned her Master, desiring to be mistress of her own lust, taking all things to herself to feed her emptiness; and she fled to the south, escaping the assaults of the Valar and the hunters of Oromë, for their vigilance had ever been to the north, and the south was long unheeded. Thence she had crept towards the light of the Blessed Realm; for she hungered for light and hated it.
In a ravine she lived, and took shape as a spider of monstrous form, weaving her black webs in a cleft in the mountains. There she sucked up all light that she could find, and spun it forth again in dark nets of strangling gloom, until no light more could come to her abode; and she was famished.- J.R.R. Tolkien, The Silmarillion, Houghton Mifflin, 1977, p. 73
This a vivid picture of unsatiated hate and eternal emptiness. Perhaps it is symbolic that Satan in Revelation is associated with the "bottomless pit" - one could say he has dug the pit himself. His long, raving rampage of distortion and destruction can lead to only one end.
"If Satan rise up against himself, and be divided, he cannot stand, but hath an end." - Mark 3:26
Evil always carries within itself the seeds of its own destruction. It cannot love, it can only hate. It does not live, but exists in a perpetual state of wraith-like unreality and decay. As Tolkien said, evil both needs and hates the light, and so drifts ever deeper into torment.
An excellent lecture on this topic is Peter Kreeft's 10 Uncommon Insights Into Evil from The Lord Of The Rings. He develops these ideas with greater insight and precision than I ever could, and helps us as Christians to better understand the reality of the cosmic warfare we are in.
I would add as a warning that there is a very distinct danger in studying evil. As Paul writes in Romans, "I would have you wise unto that which is good, and simple concerning evil." Saruman's desire for wisdom led him too deep into the enemy's council; he was ensnared and subsequently destroyed. One dare not go too near the black hole.
Though it is true we are instructed to be alert to the enemy's movements - and Paul himself in another place said of Satan that "we are not ignorant of his devices" - still our knowledge of evil ought to be chiefly a negative knowledge; that is, we ought to define evil primarily as "non-good." For evil is the absence of goodness, just as darkness is the absence of light. It is a mistake to respect darkness as a reality unto itself. Its protagonists are real - frighteningly so - but the thing itself is only death and shadow.
In the end, the surest way to recognize the counterfeit is to be intimately familiar with the real.
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