Light and Darkness have played roles in thousands of stories written by various authors, but sometimes they play vastly different roles in the stories that you read.Light Is Good
It's one of the deepest ideas in our collective consciousness. The Sun brings heat, makes plants grow, drives us to awaken, explore, work, create and give life. Gods of the Sun and Moon are associated with law, arts, the mind, justice, protection, inspiration, all the things that make up the best part of being human beings.
The lightbulb coming on over one's head is the genesis of another great idea. By light we can see the world; by extension, light lets us confront evil and destroy it, both within and without ourselves.
Almost every story does this, some more than others because It's always the light based magic that heals, never the dark. For characters with light-based powers with this connotation they can obliterate their opponents with light based powers.Dark Is Evil
Basically, darkness associated with evil, ugliness, and scary monsters. This is the reason for the naming of "The Dark Side" in Star Wars and why Evil characters often have 'dark' in front of their names. This is common, but not universal, and will vary from culture to culture.
The Symbolism is because most humans fear the dark, because dangerous animals can see better in the dark than we can, and the dark can hide other dangers. Furthermore, the fact that it's so hard to see in darkness (well for humans, anyways) has caused some of us to associate darkness with deception.
Evil is associated with deception as well, so, from Star Wars to cowboy movies, a lot of bad guys wear black hats. If you want to be even more obvious about it, give the bad guy a name that has something to do with darkness.Light Is Evil
The Light seems like a good, kindly thing, especially compared to that bad old Darkness. Whereas Darkness is usually concerned with kicking puppies, eating people, and laughing its head off, the forces of Light generally worry about nicer things, like singing with the birds, helping little old ladies cross the street, and... well, you know, being a ture Hero. Whereas the Darkness seems to revel in lawlessness and chaos, the Light tends to prefer law and order, providing for properly maintained roads, a court system, and safe cities.
However this is not always the case as the Light isn't always what it appears to be.
Sometimes, a battle with an obviously Light-wielding character will turn out to be the true villian of the story.
This often describes a beautiful, charismatic villains dressed in white and surrounded by almost religious-looking symbols. A step further features a villain who wields Light as an elemental power. Whether by technology or magic, he can bend physical light to his whims, displaying such powers as beauty-enhancing sparkles or an angelic glow, optical illusions and holograms, Mind Control through whiping out all their memories, blinding flashes, and laser beams
to explain why this character is evil and uses light you have to understand and See, that Light and Dark are both oversimplified metaphors for Good and Evil, not truly synonymous terms. Looking at the matter rationally, too much sunlight causes burns, sunstroke, and drought, not to mention skin cancer which is bad; whereas nightfall and sufficient shade provide relief and rest, which is good. So, neither physical brightness nor physical shade can be all good or all bad.
Light is Not Good is when a character is directly bad, directly villainous — and still has Light powers. Depending on the Character, they might be sympathetic, perhaps well-intentioned but horribly misguided Or they could be the most gleefully twisted people imaginable.Dark is Good
So, you have a race of people who all have black, leathery wings. They are born with the ability to shoot black, shadowy luquid out of their hands. Also, they prefer the night, and dress like dracula.
THEY MUST BE EVIL.............right?
Well...no one actually mentioned them doing evil things. In fact, they may actually be pretty good guys. It's not like they keep pet dogs exclusively for kicking. Despite any images that may have been burned into our minds, creepy appearances and harming others actually don't have much to do with each other. They are simply good people like you or me.
Unfortunately, however, people can still judge them based on their looks.
A common use for it is for the "Don't judge a book by its cover" Aesop. It can also be used to promote that Evil looks Cool except without the, uh...EVIL part.