All awakeners and monsters, small or big, have cores. Even gates have monsters as their core that need to be destroyed for them to be cleared of any energy. The only difference between these monsters and awakeners was the energy, and as someone with the same negative energy as the monsters or gates, Cheon Areum was no better than a monster with a higher intelligence and skills, granted the energy of an S+ Class boss.
It wasn't easy to contain negative energy and not be corrupted by it too. The mobs inside the gate were all beings having lost control to the negative energy, but they weren't humans to begin with.
