For those of us who define "evil" according to the moral code found in the Bible, this is not necessarily a problem.
But to the skeptic, or the atheist, the simple answer to the question would be that in the absense of a transcedent being and a transcendent moral code, then neither evil nor good can exist. Thus for the skeptic the question is a non-sequitor.
Interestingly the fruit which Adam and Eve ate from was from the tree of the knowledge of good and evil. Thus, once man ate of that tree, he became aware of what exactly good and evil were. Thus, since we are under the curse, we need not "define" either good or evil, as the knowledge of good and evil has been forever etched into the souls of all men.
So your suggestion that I define "evil" is equally a non-sequitor. It is the same as defining obscenity. You know it when you see it. --- Unless, of course, you accept Genesis only as allegory and not as literal. Then you've got a bigger problem than merely defining "evil."
If it was a non-sequitor to the skeptic then why do they use it as an argument against theism? Every skeptic knows their is a difference between good and evil but try to explain it through natural causes. It sure is a lot easier just to "gloss-over" the subject as a non-sequitor.