You should speak with the manager- I mean, Hell itself. It's Hell who determines what are they going to look like.
Seriously though:
Since Hell is the corpse of the Original Sin, Ascalon and he envied the creation of the Universal Architect, Veridatha, who considered proto-angels (who were very similar to angels) his best creation, Hell tried to mimic that, but as always, it's corrupted.
Instead of being beautiful and bringing life, they take life to sustain their beauty. A complete inversion of what the Architect wanted.