Altruistic Or Opportunistic Demon Lord.

He didn't bother to search through this fortress or the rest because of all the commotion going on. Whatever is good in them will have been looted already. The ones fighting for the right to the abyss altar weren't encouraging him either with their cowardice. They don't want to die and without demons willing to fight him, he wouldn't be able to kill them. He is fast but not fast enough to catch up to them. 

This leaves the really strong high rank demons who have something to lose if they run away. Those won't be able to run away from him.

Suddenly the divine sense that had been stalking him became active. It grabbed ambient sin energy in the air and pulled them together to form something. Aeternus ignored the change and continued to walk. He had places to be and things to do. He would have hummed a carefree tune if he had a throat and lungs for them.