I didn't say anything about the light. I said angelic beings fit the Humans:
In Warcraft 3 the Human Paladin hero unit summoned an angelic being everytime he used the resurrection spell.
In World of Warcraft, the Paladin spell, Guardian of Ancient Kings summons and angelic being.
Archbishop Benedictus, former head of the Church of Light, wore the Avatar rainment - and agelic-themed armor set.
It doesn't matter. The fact is that there were vampires officially part of the Horde:
Blood-Prince Dreven.
Blood marquess/marquis.
San'layn Neophyte.
Varimathras (Dreadlords are former Revendreth denizens).
You forget that Sylvanas recruited the Zandalari, as well. That didn't make them part of her forces.
You forget that the Forsaken and Orcs did not give up on their cultures. War and glory is still part of the Orcs after the 4th war, and will always continue to be so. Necromancy will, always, be part of the Forsaken, even after the 4th war. The Horde didn't abandon these ways. They just keep them in check for the time being.