The Biblical definition of "evil" is essentially that it is purposeful opposition to the purposes of God -- His agenda in the world and in the lives of individuals and groups.
For most of us, we experience "evil" as those things and persons who threaten our sense of "a good world" . . . our sense of "how things ought to be." And we equate this gut reaction with God's reaction to the same things and persons.
I am a fan of the writer/Christian/psychiatrist M. Scott Peck, and the first book of his that swept me into his stream of thought was People of the Lie, a collection of stories from his practice illustrating his definition of evil and his understanding of a proper response to it as Christians. I paraphrase his definition of "evil" as "the use of power by any of us to protect that which is sick within us, at the expense of the health and well-being of those around us." I think that lines up well as a practical "bottom up" understanding of the Christian definition of "evil".
Dennis Okholm did a wonderful teaching series at St. Andrew's for Advent based on N.T. Wright's book Evil and the Justice of God. Wright does an amazing job of looking at the theology of sin and evil and God's remedy for both.
We are bound up in our own perspective on what "the good life that honors God" is, as much as we think that we base it on Scripture rather than on our cultural conditioning. And out of that perspective we so often lose the ability to understand that our gut reaction to others as "evil" and to ourselves as "not evil" leaves us very vulnerable to being truly evil ourselves.
One of my favorite C.S. Lewis books is The Four Loves, in which he talks about Phileo love (non-sexual non-familial deep friendship) and the potential for evil out of it, as deeply bonded friends agree with each other on their perceptions of good and evil, and reinforce each other as they act out of those perceptions. We bond together in like-minded bands, and, like the mob, feel reassured that our chosen way of life and our ethics are safe and good, and that those who oppose us are evil.
The remedy is simple: Jesus said “And why worry about a speck in your friend’s eye when you have a log in your own? How can you think of saying to your friend, ‘Let me help you get rid of that speck in your eye,’ when you can’t see past the log in your own eye? Hypocrite! First get rid of the log in your own eye; then you will see well enough to deal with the speck in your friend’s eye."
Teach me how to live, Lord Jesus!