Original sin is not about forbidden fruit or carnal knowledge; it is the first time you look upon another with hate, not because of what they have done, but because they are.
What is evil? The flip answer, which may be more revealing than not: evil is live backwards.
"Evil," a concept, is a creature of consciousness. It is a label usefully used as a warning against the kinds of thought and consequent behaviors that are about willful destruction of a nonconstructive nature.
I have been thinking about delineating evil. I don't think it is about big events we normatively label as evil, such as Hitler, Hiroshima, et al. I think these big events are made up of a confluence of historical trends, perhaps individual psychoses and paranoias that are mutually inflamed, ingredients of fears, ambitions and the primal excitement of blood lust.
Evil, though a part of this maelstrom, comes from the little worse demons that plague us everyday. One is piqued, or in pain or in a state of carelessness more than ruthlessness, and communicates with spite, cruelty, meanness, lets the pleasure of that passing on of pain or boost of status over or soothing strike of power take control of how our interchanges take place. If we fear retribution or want to hold off guilt, we may layer our consciences with rationales about what we deserve vis-a-vis the other. We may insist we are acting righteously, to right a wrong, exact revenge, or respect divine will. We may say: Those of that group aren't worth my respect. I must protect myself from their intended violence against me which I fear I may have provoked, so I must take their power and keep them down. Or, perhaps, I was just lost in my own drama, not thinking about the consequences of my cruelty, but nothing so bad happened. Or, all my friends, my neighbors, my kingroup do these things to those unlike them. I am merely being loyal to my own against a potential threat. Bit by tiny bit terrible consequences so easily build. Evil may be a very little thing.
I am thinking of evil as occupying the extreme negative grid of both the compassion and selfishness scales. Sending out bad feeling like a curse into the other does not diminish it in the sender, but rather increases that tendency as practicing deepens neural patterns. It becomes an ill wind blowing the disease about, a vector of infection.
What is the cure? I don't know about the big picture, but in the sense of being the change you want to see, and becoming the person you want to see in the mirror: Look at what you are doing! Now, don't turn the cruelty inward. Look at your bumbling stupid self and give a big psychic hug released in lots of laughter. Then do what it takes to make it right. Graciously apologize, including humble acceptance of responsibility for consequences. Open to the outward manifestation of your inner demons and find, most likely, that they are not who you imagined. Open yourself to your own life and where it takes you, who you encounter, what you come to understand. Is this goodness? Is this love? Maybe it is living forward.