We know that the Dusk are forces of the Twighlight (As you explained) But they also appear to have a lot of symbols of the light (They are white for example, instead of black) which leads me to believe that they are forces of currupted light, or light that was currupted by the darkness. Now the Soul is, in essence, pure, but our hearts sometimes lead our souls astray (This is pure theory by the way). Now if the soul is pure light, then what happens when the misleading heart is taken away from it, and the soul is bathed into darkness (As Sora's was in KH1). My theory is that after becoming fully currupt, it becomes a Dusk, which as I said before, is currupted light, or in essence, a currupted soul. Now this dusk seeks to destroy the world, and turn it to darkness, or in symbolicism, make the day turn into night (Dusk).