If we are looking for definition exactly on what the definition of evil is, then why not start on what the definition of good is?
God made everything, and said it is good. He also said it isn’t good that man should be alone.
But why? What was the necessity that God had to create? And, what was it He found that everything He made was good? And, why wasn’t it good for man to be alone? Afterall, God is there. Man wasn’t alone in Divine Covenant Relationship with God. So exactly what did God mean by saying, “man should not be alone.”
Well, as Saint John the Apostle describes, God is love. Thus, charity. We know God in Three Person’s: The Father, the Son, and the Holy Spirit.
Adam, in order to really be in God’s image, and after His likeness, could not be alone. So even though with everything God made, He said/saw it was good. But man being alone, definitely no. It wasn’t good. Because, man is made for more. Not just this one man (Adam( before God. But others. So Adam had to share His Divine relationship with God, the Holy Trinity with another, Eve. But then again, He said to them: “Be fruitful and multiply (i.e. that love in Divine Covenant Communion.)” God thus made man in His image, and after His likeness in Love. And found it was good. And so He rested on the 7th day.
What is contrary to that love, is evil. Hence, when the devil looked for the “weaker sex.” And no, I am not saying a woman is a weaker sex. But the devil did believe her to be that way. That’s why he tempted her directly, not Adam. With that, he opposed that Divine Love and Communion with God, in their relationship, by telling her (essentially saying God is a liar.) You sure will not die (you will live forever…) You will become like gods (plurality, not just one, but many.) Thus, the devil is getting Eve to deny and destroy her family, in Divine Holy Communion. For, by denying herself as a human being, made in God’s image and likeness, she is tempted to become like God. Interestingly, by being in that Divine Love and Relationship, she is already like God Himself, in the image and state of the Holy Trinity. In fact, she was created. And why would the devil tell her to become something she was already sharing in by virtue of being made in God’s image and after His likeness, did the devil tempt her? Because, he didn’t believe they were made in His image and after His likeness. And so the devil wanted to test and see. He didn’t believe God could do this.
So the long winded way I can answer that question is evil is which opposes love. Especially God’s Divinely ordered love. Meaning anything opposing the nature in which something is to be and is. By taking something out from what it is, or destroying it, or changing it, is evil. It opposes.