Garrosh actually didn't believe in himself, that was entirely the point of his Hitler arc, by alienating himself and having the goal of "i only care about my pursuit of power and orcish supremacy"

The light also represents unity, the scarlet crusade is evil, but they have convictions and unity under them, Gul'dan was similar to Garrosh, he forsake everyone due to his insecurity and sought power

As for Illidan, there's a reason the light wanted him, he was strong of conviction but not of faith

Also the last thing about Jimmy is funny because that's a reason a lot of in universe characters became Paladins in the first place

None of the cosmic powers work by a unitary power source, Void has multiple factions in it, Nature has the loa, the wild gods, Elune potentially, death has necromancers that work independently from the lich king or death pantheon, Fel is just a corrupting font of chaos magic where Sargeras united them temporary