Strangely enough I just explained this a couple of weeks ago to a friend.
I took a numerological approach.
And I'll justify why I'm doing it here for clarifications sake. Math is a good way to describe reality, because it is pretty much unambiguous.
Ok. Good and evil depend on perspective. What's good for me might not be good for you.
So good for me = +1 bad for me or evil is -1. (from my perspective.)
If good = +1 and evil = -1 then:
-1 + -1 = -2
So you cannot defeat evil with more evil.
An example would be revenge. You hurt me and in order to get back at you I hurt you. And then you hurt me again. Until eventually either one or both of us are so hurt that we are cripples. A lot of -1.
A +1 would be peace and harmony, synergy, cooperation. But that would be a +1.
What you can do however is multiply by -1 instead of adding -1
Then you have -1 * -1 = 1 and you are back to good. Depending on the perspective.
What happens when you multiply by -1? The direction changes. It's the same thing, like a road sign, but now it points in the different direction. That means that either you have changed your own position and perspective on the issue and can see the good in it now. Or you have somehow turned the "bad" thing around to point in the "right", "good" direction.