Any moral or ethical system tends to be derived ultimately from religion. What the gods like you doing is defined as 'good' and what they hate you doing is 'evil'.
Without gods to make things easier, one is forced to judge from the rules of one's society, wherein 'good' is doing what society as a whole would approve of, and 'evil' is doing something of which society would greatly disapprove.
In either case, it is usually far simpler in any game system to define 'good' and 'evil' by the normal Judeo-Christian standards that the West has long used as the underpinning of its society.
Modern people may or may agree with those standards, but they at least know what they are...
There never have been any objective standards for 'good' or 'evil'.
