Evil essentially boils down to "You're doing the exact opposite of what it says in my holy book, ergo, you are not a good person".
All morality is a religious concept. A lot of people seem to think this is the other way around, but it isn't. You can look at cultural concepts of what's "right" and what's "wrong", and you'll see that they're usually pretty on-par with the local religious beliefs.
This is usually derived from empathy, simple science, or occasionally, the need for a group to impose control on another. If someone does something we wouldn't want done to ourselves (murder, rape, etc), then this is wrong, so we write rules against it. If we find out that eating, or example, uncooked pork causes us to become ill, then we write rules against it. Finally, if we notice that other people aren't following our rules, we write rules against that. Which leads us to stuff like the ten commandments.
This is why something, such as cannibalism, is seen as one of the greatest evils in cultures such as our own, however, to the Mayans, it was seen as a necessity.
Simply put: Evil is what people say it is.