"Good" and "evil" are just descriptors for the same thing on a certain scale.
Consider this:
There's "temperature" yet it may be hot or cold. It can't be both, those are mutually exclusive; even though both hot and cold are the same class of things (temperatures).
Good and evil are also same class of things, but very different extremes. To say they are the same because some arbitrary law is blasphemy.
Is it possible for there to be only good, or is evil necessary?
Try hard, it's a loaded question about your credibility, in case you can't tell.