One of the questions for the ages. First thought was I don’t know.Then I got an epiphany, evil only resides in the realm of man, not in mother nature. Trees, birds, insects and animals aren’t evil. They co-exist in a balanced natural world. It would still be balanced and natural if the realm of man did not intrude so much upon it. We are the carriers of evil, it is within us and thankfully we, for the most part, do not even acknowledge its existence within us. However, when fear, hate and intolerance fill our hearts and mind, evil is given an ample playground within us to grow. We are capable of so much good and so much bad. Evil actions are intentional and designed to inflict pain on others,there are many things thrown at us in nature that cause us various forms of pain, but those events were never intentional, aimed at us. Therefore they can’t be evil. Evil is only in our human world, in our hearts, mind and psyche. Until the day the humans no longer exist on this planet, evil will walk this earth. The only hope that might exorcise evil from mankind would be for everyone to voluntarily come into acceptance of the other. Without fear, hate and intolerance, evil can’t exist.
I don’t know if I answered the question correctly but I think the direct answer to the why part is that without evil there wouldn’t be good? I think I’m thinking of the question in the same way as I think of happiness. There has to be sadness for there to be happiness or you wouldn’t know the difference. I’m not quite sure that if you didn’t have evil you wouldn’t know good. I still think that evil is dependent on the emotions of fear, hate and intolerance. Once you banish those emotions, evil hasn’t a leg to stand on.