Upon learning of the Bliss Society's plan, Lumian's immediate assumption was that Susanna had made a crucial mistake.
What lay sealed within him wasn't just corruption at an angelic level, but an actual Angel!
The former lacked self-awareness and reacted on instinct alone. Without undoing the seal and reconnecting it with its true form, it was like a cache of explosives temporarily without a detonator. While there was still a possibility of explosion, Susanna and the other heretics believed they could manage the situation.
By employing the right method, utilizing the isolated environment within the Tree of Shadow, arranging the necessary rituals, and harnessing the evil god's gaze during the sacrificial ceremony, they could break the seal and offer it as a sacrifice to the Mother Tree of Desire, ensuring the angelic corruption wouldn't pose a threat.