My own definition (to be incorporated into forthcoming manifesto of evil)

[overlord_mordax]

evil is

A:  acts committed entirely for self profit, with little or no regard to the feelings or well-being of others.

B:  acts committed in opposition to, defiance of, or lacking any regard for, established social and cultural standards, practices, laws, expectations, etc, especially, but not limited to, those governing 'moral' behavior.

C: any combination of the above two.