One of the main characteristics of the universe is its dualism. Everything appears to come in twos, everything from binary numbers, to the Cartesian mind and body, heaven and hell as well as good and evil of the Christian religion. Even matter has its counterpart in the form of antimatter. It seems that after all, the yin and yang of Taoism applies to all creation in the universe.
We can say that evil is the opposite of good since the one cannot really exist without the other. But what is evil? Evil can be seen as that which creates disruption or disorder. When the universe is in balance and harmony, there is peace and quiet, but the moment something intrudes upon and shakes up the state of equilibrium, there is chaos.
From a Christian point of view, we can say that Eden was in harmony until the serpent interfered and made Adam and Eve revolt against the natural and divine order. Evil is that part that disrespects any given situation to enforce its own will and desire. In other words, the serpent wished to create havoc in times of peace.
Traditionally, the blame has been on Eve, which is rather unfair. Although we can say that she acted out of her own free will, we are underestimating the power of suggestion. The serpent sounded convincing and would have persuaded any other being as well. Some claim that the serpent used Eve as a messenger to get to Adam. But Adam was not much stronger nor wiser because he could have easily dismissed and refuted her arguments and not given in to temptation.
So evil existed or came about through the beguiling words of the serpent. But if paradise was in harmony, then how could or rather would such a being exist in the first place? Why would God allow evil to enter his very own kingdom? Who would willingly keep a snake in the garden where their children play?
It may lead us to the following question: Is such disruption always bad? Adam and Eve were chased out of paradise because of their actions and were cursed with death. But without the notion of original sin, there would have been no humanity. We would not have existed from a biblical point of view. And despite wrongdoing and undoing in the world, I do not think that humanity is inherently evil. Misguided maybe, but at the same time, there is a lot of good within each human being and even more potential.
Just like any other part of nature, we are also caught up in duality. We have a good and evil side and both are equally part of who we are. In fact, we owe our existence to it in a figurative way, as Adam and Eve demonstrate, as well as biologically, since without sex there would have been no procreation. The world is not merely a matter of black and white as actions deemed good may have negative consequences, the same way so-called negative acts may lead to positive results.
Because of duality, one thing differentiates itself from the other and in fact it needs and relies on the other to exist. Does the beautiful not exist because of the ugly? Does the righteous man or woman not exist because of the wicked person? Is the existence of death not necessary for the creation of life? Does harmony not exist because of its counterpoint chaos? And finally, does God not exist because of the Devil? In a world where there is no evil, would God's existence not be redundant?