Belial is referred to as "the Angel of Darkness," demon of false council and the worthless, and Lord of the eathly plane.
So my interpretation was going for something like "What if an angel became mutated with physicality?" going off the notion of classic biblical angels being wholly ethereal entities. Obviously, the transformation from fallen angel to Demon outright didn't go so well for Belial, but that's just his luck, as one of the weakest among the Devil Princes.
Is he weakest in might? Hell no! He's a fallen Angel! But he's always looked down upon, even by his peers, for his "cowardly" tactics and seeming affinity towards the bastards and baseborn of the world.
As a Fallen, Holy magic doesn't hurt as bad as with other demons, but Acid seems to do the trick as it dissolves the exposed flesh that still quivers towards complete mutation to this day, though failingly.