Shorter version ...
Evil is nothing. It is not a thing that has existence. It is an action of something that is a thing. When I do something that is not good, then I am doing something that is evil, but evil then is an activity of some being. It has no being of itself.
Augustine and Thomas Aquinas use the words “negation” and “privation” to define evil. Negation talks in terms of what something is not. For example, we say God is infinite which means He is not finite. Evil in this sense can only be defined against the backdrop of what is good. In biblical terms, evil is defined by words like ungodliness, unrighteousness, injustice, so that the term is used as the negation, the opposite of the positive thing that is being affirmed, so that injustice or un-justness can only be understood against the previous concept of justice. Unrighteousness can only be recognized as unrighteousness against the background of righteousness as the standard by which unrighteousness can be recognized and can be defined. Evil is parasitic. It can only survive in a host.
A “privation” is some sort of lack of or deficiency. This occurs when you don’t get something you need (necessary/essential) as opposed to something you want. WCF Shorter Catechism on “What is sin”: sin is “any want of conformity to or transgression of the law of God.” This is a definition of evil in terms of a lack or privation of a want of conformity. Righteousness involves conforming to the law of God, doing what God commands. Sin enters in when we fail to do what God commands, and we fail to conform to His standards of what is righteous. On the one hand the catechism says that sin is a want of conformity to, which is a kind of privation, or transgression of the law of God.
Reformers said the sin is a negation or evil is negation, evil is privation, but it is also ‘privatio actuosa’ meaning that though evil is not something that exists in the of itself, it is real, and its effects and its impact are devastating because real beings act out real evil though evil is not independent, nevertheless it is real. R.C. Sproul