Demons Are Selfish. It Is Their Virtue.

He would have gotten away too if Viron hadn't started that last spell. He didn't know what the spell could do but he knew it was bad. Nothing that accumulates so much mana and creates such a commotion in the fluctuations of ambient sin energy can be good. It meant that whatever Viron was doing, he had to get a lot of external help to achieve it. It couldn't be good.

So he returned to disrupt the casting. It was then that he noticed that Viron's mana was low and his status was mentally drained. It was an opportunity he couldn't miss. He even used the knowledge of the weakness of the spell that he had been hiding. 

He had been pretending to be unable to deal with the fireball spell because he couldn't take advantage of it earlier. Viron might be able to cast spells, but it is only barely. There are some vulnerabilities in the spell structure that he can take advantage of by using Chaos energy to destabilize them.