For other races, materials are precious, but slaves are aplenty. Killing some slaves to refine soul food is a basic operation, and they don't even need to find a temporary excuse to exterminate a few slave tribes—the warehouse already has plenty in stock.
Besides, it is also to prevent providing aid to the enemy.
Even when refining pure soul food, it will still retain traces of someone else's soul origin and spirituality. If too much is absorbed at once, it can cause one's own soul to become impure, hindering the path to the future.
As for mutants and devotees of the Evil God, they can absorb souls at will?
Nonsense, do ruins fear collapse?
Having embarked on the path of evil, they have already prepared for the possibility of dying at any moment. Very few make it to the end, and perhaps some become Ancient Giants, or even Ecological Lords, commanding great influence.
But that's just survivor bias.
