Zeliek and the scarlets are being mind controlled by an outside force.They could have changed the spell visuals to be shadowy or physical like Vampiric Embrace or Shield Slam - but they didn't. For both.
They could have given them shadowy icons - Death Knight icons - but they didn't. For both.
They could have made them do Shadow healing or Shadow damage, but they didn't. For both.
Someone somewhere intended for these Undead to use not just one holy spell in spell effect, and icon, and magic properties -- but TWO of them.
If a mage yells, "Fireball!" and hurls an icy-looking spell, that has frost icons, and deals frost damage, and then yells, "Ice Barrier!" And summons an icy-looking barrier, with a frost icon, which turns us blue and slows us when we attack it, are we supposed to just say: "Y'know, he said Fireball the first time... I'm convinced!"
Anecdote aside, if it's okay for Zeliek, the Scarlet Crusade, and random undead Scoruge NPCs to use The Light because they're insane, why not players?
You can't say it's because players can't be insane, because players can be Shadow Priests, who literally wield a mechanic called Insanity. The first Paladins were trained by Uther the Lightbringer, straight out of Lordaeron. The most devout and powerful paladins fell there. I have trouble believing some no-name Scarlet Crusade zealots have more faith than those that trained under Uther the Lightbringer.
does not help your case.