People of goodwill can see evil everywhere. It's present in corrupt politics, media and in the Church. But evil can also be seen within families and in neighborhoods. All levels of society are saturated with evil, yet few people understand what evil actually is.
The Church gives clarity as to the nature of evil. Saint Thomas Aquinas explains that evil is not a thing that, properly speaking, exists. Instead, the saintly doctor notes that evil is a lack of some good that ought to exist.
Good is defined as things being exactly the way God meant them to be. In other words, when a thing does what it's supposed to do, it is good. God designed all of creation to have an end or a purpose — namely, to glorify Him.
Evil was introduced into creation with the fall of angels, when some of the angelic host chose to rebel against God, becoming fallen angels, i.e., demons.
It also came with the fall of mankind. After God finished creating the natural world, He declared His creation "good," but then Adam and Eve sinned, and evil poisoned the created world. The result was mankind's tendency to love sin rather than God, along with the introduction of disorder to the natural world.
Nevertheless, evil is not to be understood as a power that pushes out or destroys goodness, nor is it a power that fights against goodness. It is an absence of goodness, meaning that creatures are not doing everything God created them to do; they are not cooperating with His glorious plan for them.
God gave both men and angelic beings free will. The reason some of these creatures have acted in disorderly ways, however, is that they've abused their free will to prioritize themselves over God. Each angel chose either to conform himself to God's dominion or to rebel against it. Members of the human race do the same thing, over the course of our natural lives.
Despite the rebellion of angels and mankind, God is more powerful than evil. He even takes evil and turns it into good for those people who ask for His grace. After all, God took the greatest evil — His crucifixion — and turned it into the means of salvation for sinners.
Learn more by watching The Download—America's Great Divorce.