JL's take is that evil and good are just a concept, rather than a law.
There's an old saying that we only know good because of evil, but I don't buy into that silliness - we are driven to many things, most of which can be overcome by our minds...including the need to believe that goodness exists because of the evil. Warmth is capable of existing without hate.
In terms of the usefulness of the word 'evil' - if it did not exist, another word would exist in it's place. Humans find the word useful in power struggles too.