My personal take:
Evil is a cultural construct - not necessarily a catch-all, but evil is defined by society, and each person within the society decides how they will define evil. What is determined to be evil by society as a while is sort of a "majority rules" kind of system. One example of this is sex: many things are considered acceptable today that were previously considered evil or, at least, shameful.
That being said, there are simultaneously certain 'self-evident truths' when it comes to what is good vs. what is evil. For instance, just because many in early societies felt that slavery was justifiable, or even good, that didn't make it good. The act was justified in the minds of the persons carrying out the acts, but those acts were by their nature evil. The same can be said of the Nazis with the Holocaust or the Khmer Rouge and the Killing Fields, among other examples. There is something in human nature that finds certain acts to be evil, and while there have been instances of societies that have not defined those acts as evil, there is usually some sort of over-riding ideology in those societies that the societies' leaders use to indoctrinate the citizenry into believing that evil acts are justified or good.
So, to simplify things a bit:
There are basically two kinds of evil: evil that is defined by society, and evil that is defined by "natural law," or "God's law."
Let every nation know, whether it wishes us well or ill, that we shall pay any price, bear any burden, meet any hardship, support any friend, oppose any foe, in order to assure the survival and the success of liberty.
~John F. Kennedy