"Darkness cannot drive out darkness; only light can do that. Hate cannot drive out hate; only love can do that." – Martin Luther King, Jr.
Light and dark are opposing forces. (Duh.) Darkness is often associated with hate, and in Beautiful Creatures, things are no different. Lena's mother, Sarafine, is a Dark Caster filled with hatred and rage. Lena, on the other hand, is motivated primarily by love—her love for Ethan and her love for her Uncle Macon and the rest of her family.
In the end, Lena's love for Ethan drives Sarafine and her other Dark Caster relatives away. But she doesn't choose Light. Instead, she chooses, well, not to choose. Lena becomes a combination of Dark and Light. By not choosing to go Light, she's only temporarily diverted Sarafine. It's kind of like shining a flashlight in your eyes. You can't see for a minute, but after a few blinks your sight returns. Sarafine will be back.
The words of the epigraph make sense, then. Lena will have to be full-on Light in order to drive away darkness. But why Martin Luther King, Jr.? Other than the fact that he's eloquent, persuasive, and just plain awesome, why do you think the authors chose this guy to open their book?