I'm a new member, and this is my first post. Couldn't resist the question.
Wow, so many "good" thoughts.
I don't think evil is a mesurable quantity in itself, but rather the absence of the quantity of good in an action, person, etc. Evil is thus not an equal opposite, but rather the sad absence of goodness. Goodness is everything from being kind and loving, to beauty, joy, intelegence, health, or energy. Good is when things are "right." So, evil is always the twisting or perverting of what is good, not a force in and of itself. For someone to be completely evil, they would have to have no friends, no honor, no health, no strength, no joy. Few evil characters take it to that extreme, though it would almost be comical to see one try.
I do think everyone has some evil in them simply because no one is perfect in their goodness. But it is good to try to end evil in all it's forms.
I don't think good and evil are always relative. Some things are simply wrong and some are simply right.
Our thoughts, feelings, and beliefs often lead us into evil. I have a had time with the insanity as evil idea because even guilt can be over ridden if we "feel" or "believe" strongly enough. In fact, I think evil people rarely realize that they are evil! It's very easy to close your eyes to goodness when you "believe" or "feel" your protecting yourself or those you love. I think the main measuring stick is, as one wise man said, "do to others as you would have them do to you." And really, that takes a lot of work. The principle is realizing what is for the good of all, and to do that you have to deeply know and accept others.
The characters in the stories I'm working on are always changing that quantity of good by the power of their choices. Evil is an easy, downward movement, while good is a difficult process of admitting your faults and trying to change them, idealy with the help of others. It's like the baby that was brought up earlier: we are predisposed towards our own interests and we have to be disciplined and taught by ourselves or others to be good.
Last of all, I think there must be, in a story or in real life, a higher power which IS completely good, encompassing all creativity, beauty, joy, power, satisfaction, etc. It would have to be all knowing because only an all knowing being could see all results and outcomes to know if an outcome is completely good or not. I think that without at least the belief in such a being, our entire existance would end up being relative and meaningless and we would have no measuring stick for existance aside from our own, horribly flawed selves.
So, the evil power behind everything in my stories, is simply a powerful, but created, entity, who has rebelled against... well... goodness, which was the nature of its creator. Yet he will only be in the far background.
And that just opens up another whole can of worms, but hopefully it was helpful!
Darn it... I ment for that to be a SHORT response. Sorry about that.