Light has no monopoly on angelic aesthetic.
Uther was send to Bastion not because he was dedicated to Light but because he was a soul with strong dedication.
Said dedication was to be used to serve the Purpose.
Zovaal's ideas for death's purpose in the cosmic order are likely too radical and extreme and the rest of Shadowlands have decided they want none of it. That's why they locked his crazy ass in the Maw.