Why do Death Knights eyes glow?
Simple, undead in the Warcraft universe have blue, glowing eyes. The more powerful the undead, the harder the eyes glow. That's why a ghouls eyes are pretty much non-glowing, and why Arthas (aka The Lich King) eyes produce a stream of fog. It's kind of the measurement of how powerful an undead is in Warcraft.
Can Death Knights have red eyes?
Yet DKs are only allowed blue even though there are DKs with yellow, red and green eyes. We have quite literally greater eye color diversity among DK npcs than we have forsaken NPCs.
Can a Death Knight use the light?
The Death Knights were created to destroy all life, not necessarily the Light. The Light just arose to oppose them. The Naaru aren't really involved in the creation process anymore. T'Paartos, for example, becomes Lightforged through his trials alone, with no involvement from any Naaru.