Just think of something that you feel is good. Now who is it good for. Probably for yourselves,people related to you or organisations you belong to etc.. What does that good do to some others? May be ,just may be, it can have an evil impact on some other people. Thus good/evil can have a limited reach. This idea is very well expressed in the book The Oath of the Vayuputras written by Amish Tripathi. In the book the author explains how something thought to be an ultimate good turned out to be greatest evil to the world. It is an incredible book which deals with a lot more stuff .
So back to the point. It is clear that good and evil is not separable. So our efforts to eliminate evil is meaningless. Isn't it more sensible to create balance between good and evil, rather than trying to eliminate evil!! The harmony can be found if perfect balance is achieved between good and evil, like the Yin & Yan.
Good and evil is not a physical thing, it is an energy we feel. We all know the fact about energy, it can neither be created nor destroyed. So how can we expect to destroy evil. As the fact states it can only be transformed from one form to another. So good can be changed to evil and vice versa. Finding a balance between this transformation is all that we can do.