What is evil?

A recent thread got me to thinking.

Some see evil as a positive force. In The Book of Job, Satan tells the Lord that he has just returned "from walking to and fro upon the earth." People who perform exorcisms believe in evil in this sense.

Then there is the absence of good view, by which evil is not a something but a nothing, so there is no evil in a that sense.

And then there is the Socrates/Buddha view that evil is just ignorance.

There is also a view that circumstances can make us do evil, so it is really just a force of nature. Given the right circumstances, presto: evil happens.

And then there is the "Evil is Illness" theory. This is the most common in the modern liberal state. At least up to a point, we prefer to order treatment rather than punishment, at both the societal and personal levels.