They're called Non-existant ones, or Nobody, not "Mindless" because losing your mind doesn't mean anything in the KH universe. It just means your crazy XP
There are three things:
Heart, Body, and Soul.
The Body and Soul cannot seperate from each other, but the Heart can. A body and Soul can sustain life, because the Body is alive because the Soul gives it life. If the heart is strong enough, the Heart can live on it's own too, but it would have no vessel to hold itself in.
Ansem is an example of a Heart that was too strong to become a Heartless, but cast away his Body and Soul. His Heart was strong enough to exist on it's own... he just had no form. Therefore Ansem wore his brown coat.
Ansem used Riku as a vessel, by manipulating Riku into opening his heart to the darkness.
Either way, when the Heart leaves to become a heartless, the body and soul can still sustain life. And as said in Ansem report 11 and up, the Body and Soul are sent to another realm of some sort, where it becomes a Nobody.
When a person of a strong Heart, casts off thier body, or become a Heartless, the Body and Soul left behind, are somehow different from the Body and Soul left behind by an average person. They continue life from the day the Heart left, as if it never happened.
Whether they keep thier memories from thier other existance, we do not know. I speculate that they do not, seeing as Memories are in our hearts and therefore follow the Heart. Due to the fact that they have no Heart, and technically aren't thier own existance seeing how they aren't what they used to be, yet still live on, they couldn't necessarily be deemed existant, hence the name Non-Existant one.
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Explanation of Heartless, Nobody, Non-existant one's, Shells, whatever you want to call it, provided by me.
Now, onto showing you the flaws in your theory. One being shrouded in pure light is not a bad thing. Sora is the Hero of Light, he walks in the Realm of Light and is all things pure and good, does this make him evil?
No.
The quote "The closer you get to light, the greater your shadow become's" is one of my favorite quotes. It does not mean that seeking the light leads only to Darknss, or that following the light is the same exact thing as following the Darkness...
I like this quote because it can be seen in many ways,
The more you walk towards the metaphorical light (Good, your goals, the right thing to do, heaven in a sence, truthfullness) the more the bad things will try and stop you (Devil, people with bad intentions, manipulation, things trying to send you astray)...
Or, in the sence of actualy light, the closer you are to the metaphorical light, the bigger the darkness behind you is. But if you can face that darkness growing behind you, then you can reach your light.
Just because going towards the light means getting a shadow behind you, doesn't mean that reaching that light isn't worth the battle with the darkness.
The closer you are to the light, the greater the darkness may seem....