A Purpose of the System.

He was born quite healthy for a demon fledgling. He was bigger and meaner looking than them. His claws were like obsidian and his skin was just as tough. But he couldn't move. His body grew gaunt and thin after years of starvation and deprivation. When he refused to die, his body began subsisting on the very Chaos energy that was killing him.

His body experienced irreversible changes as it became more in tune with chaos. Demons can adapt and evolve as long as they have enough energy to do it. His demon body decided to side with chaos energy and evolve. 

His body became capable of withstanding chaos. Imagine that. Now he has both the chaos energy and his body as his enemies. Even if he somehow deals with the mark of sin in his soul, his body will be there waiting for a second round. His situation was becoming more and more hopeless.