Too many see evil as a vague concept rather than a stark reality demonstrating the rejection of God's goodness
According to St. Thomas Aquinas, evil is not a thing in and of itself, but rather “the absence of a good” (Source). For instance, coldness is felt when there is a loss of heat. Analogously, evil is present when the natural good that should be apparent is absent or disordered.
Until the fall of the angels, there was no evil, because no one or nothing was lacking in goodness, as God had intended. The Catechism tells us that Lucifer and the other angels were created good, but that the fall “consists in the free choice of these created spirits, who radically and irrevocably rejected God and his reign.”
When God created the world, He saw that it and all of nature were “good.” Adam and Eve, however, chose evil over the Lord, at which point evil entered the earth. Due to this original sin, disorder entered even nature itself, and created in the human race a proclivity towards sinfulness (concupiscence).
Evil exists primarily through the many ways that human beings put their own disordered desires above the law of God. Indeed, the amount of suffering caused by choosing evil has been rampant through world history: mass warfare, broken families, aborted children and corrupt totalitarian regimes are just a few of the evil results of human sinfulness.
In his goodness, God has given human beings the free will to embrace or reject his love. Despite the many times each of us has chosen evil, the Lord’s mercy is there for those who repent and turn back to him. Ultimately, the Lord conquered death and evil through his Passion and Resurrection. It is for this reason that we can have confidence that the battle against evil is won and that our hope is rightly in the Lord. Echoing the words of St. Augustine, we can say that “We are an Easter people and Alleluia is our song.”