DARKNESS is the absence of LIGHT. Always. Never the other way around. LIGHT is LIGHT-- regardless of the existence of darkness.
The same way that EVIL is the absence of GOOD. But GOOD doesn't necessarily mean the absence of EVIL. GOOD is GOOD-- regardless, and ironically, because of the existence of EVIL.
Man is educated to cultivate GOOD and extinguish EVIL. And it's just ironic and so damn heartbreaking when man finds himself doing EVIL for the sake of trying his best to be good.