How can light save humanity? Light is inanimate. It is an attribute not a source. The only way that light can save humanity is via the source of the light. The light of the world is Jesus.
Evil is that which opposes facts/reality/truth. It has no independent existence and is delusional inasmuch as when confronted with truth it MUST dissipate since it lacks substance.
Facts are existence and events - that which is and that which occurs. Call it reality if you like. Truth is the perfect communication of facts/reality. Evil is lies/imperfect communication of facts/reality.