Being a Demonic Priest is less about worship and piety, and more about pure, unbridled hatred. Hatred of everything really, but especially vitriolic hatred of those self-righteous do-gooders from the Empire, Forest and Temple of Light. This made it very easy for Abengard to rise to the top of the Order of Chaos. He didn’t have to read or study anything, or pray, or meditate, or even preach. He just had to hate, which is something that’s always come very naturally to him. He even hates the Order of Chaos itself, mainly for its stupid, self-contradictory name.
His very existence is actually a controversial matter among Demons. What would they, lords of destruction and chaos that they are, need a healer for? Many Demons refuse to regard Abengard as anything other than an embarrassment. But the healing powers of the Demonic High Priest are actually an indirect, secondary consequence of his true talent, which is increasing the damage inflicted by everyone in his whole team. That his allies are also able to life-steal some of the damage they inflict while under the spell is just a little added bonus.