While Neveras would not seem to be afflicted by such little emotions of Mortals because he is an Ancient, a god, or Archangel, his creation forced him to have every emotion, every feeling, and even every condition of Mortals. So an example could be... he could go madly insane and hysterical, but he'd probably be good. Or he could go stone cold and hollow, and become evil. But here's the problem, he's unpredictable because they could mix. He could become hysterical and senselessly murder. Hell, he could become anything. How would you feel spending centuries inside a cold, dark pitch-black void, being chained and held down to a non-existent floor? He just snapped, is all.

While it may seem more than just beneficial to lock him away in that cage... they should have made sure to keep him there. Because now the deed is done, and he's out.