Evil is what is morally wrong. Morals are what man defines. Outside of man, evil doesn't exist.
Logically, there is good or not good. The word for not good is evil. Use of the word good, in context of morals, is more of an adjective than a noun. Good as an adjective and good as a noun carry basically simular meanings and connotations.
Seems erroneous to me, but most people lean heavily on the adjective use of the word God and equivocate it for the noun use of the word good. Perhaps to say, noun use of the word good is ignored because of lacking the good understanding of the word good.
So, you are somewhat correct. If something, a good, does not exist then there is the absents or the non-existants to be called an evil.