As Leo said, evil is an IDEA, and is arbitrary, subjective, and a matter of opinion. There is no tanglible physical object upon which some concepts are based (eg Mt Everest, water, etc).
Consider this example:
To a rat, the house cat that is eating it alive is the pure epitome of evil, and every cell of the rat's body is trying to escape the grips of the cat's claws and deadly fangs. If you asked the rat (or read the "rat Bible"), their Satan would be feline.
To the owner of the cat, the feline isn't evil: he bought the cat specifically to deal with a rodent infestation in his house, and to him, the cat is not evil, but doing his will.
However, did I mention that the rat is actually the Morro Bay kangaroo rat, an endangered species that is protected by Federal law?
Now, suppose the cat's owner allowed the cat to freely wander the neighborhood, and the cat hunted kangaroo rats on land specifically set aside as habitat for their populations to recover?
To the authorities who manage the habitat (Bureau of Land Management), the cat's OWNER is the bad guy, deserving of punishment for allowing his cat to wander free. Ask an environmentalist, and they'll say the OWNER is EVIL for allowing his cat to driving an endangered species closer to extinction, and the owner is demonized.
See how that works? There is no objective EVIL. Your perspective likely changed, depending on the facts offered: there is no objective or universal evil in the example. Evil is simply what we all AGREE it to be, and some are willing to believe in imaginary evil figures when there's no reason to do so. Fortunately, REALITY is not open to a show of hands, where the majority agreeing upon the existence of Santa Claus doesn't mean that he exists.
The subjective nature of evil doesn't mean that humanity hasn't created LAWS which assume the concept of "good/bad" (or "desirable/undesirable") actions, which can be defined in terms of social contracts, and contributing to social harmony. But to assume that we need to burden our thinking with religious ideology (where "sin" and "evil" are colored by religious connotations, presupposing an existence of a moral law-giver) is foolish.