If you use evil with a capital E, generally it's a personification for a given religion. A great evil, a great enemy, a great satan... I generally interpret such a use as being an attempt to unify or coerce the audience.
Calling specific people evil is just leveraging a common archetype to simplify communication. You say they're evil to bundle up malicious intent, willingness to harm, and disregard for others all in one and then a sort of force multiplier to indicate a generally higher degree than is socially acceptable.
I think evil makes for a nice story or a nice metaphor, but its influence ends there with regard to a universal force. It's great for literature.
The
Now that's profound. ;-) I can only counter that with the less restrictive A.