I'd say Evil is whenever someone is willingly advancing himself at the cost of another.
So knowingly making the world a worse place for someone else, and not caring, in order to make it a little better for yourself.
This can include doing nothing where something is required. I remember a saying stating that in order for evil to triumph good men don't have to turn evil themselves....all they have to do is stand aside and let it happen.
I do not believe Evil in itself is a personage, however being deeply religious I do believe in a person that is actively opposing every good thing. (This is not the same)