God, who loves us unconditionally, gave us free will, the option to do good or to do the opposite of good (evil). The choice of practicing evil results in a condition known as sin. Evil is the action, and sin is the result of that action (separation from good).
As a practicing Christian, I think the Bible, where the teachings of my faith reside, defines the unseen reality behind evil as Satan along with the demons (or “angels” in some translations) who follow him. I know this sounds like the stuff of fairy tales, but I think a case can be made that the Bible’s teaching about Satan and the creation account of humankind’s fall in Genesis 3 is as viable an option as any that defines the source of evil in this world.
“Do you renounce the devil and all the forces that defy God? ... Do you renounce the powers of this world that rebel against God? ... Do you renounce the ways of sin that draw you from God?” These three questions are asked in Baptism and Confirmation services in my Lutheran church and probably in similar services in some other Christian churches. To each question, those assembled are asked to respond, “I renounce them.”