There are two recommended ways to deal with the concept of evil in your campaign: the objective approach and the relative approach. This second version is variant approach and should be used with some caution.

The objective approach
This is the straightforward approach taken in the D&D game, and it is the one stressed in this book as well. From this frame of reference, evil can be judged objectively. The evil nature of a creature, act, or item isn't relative to the person observing it; it just evil or isn't.