If you read the lore that the calcified fragments have, the Hive came to become so evil and dark because of the 4 Worm Gods from the planet Fundament. They gave the Hive immortality and powers directly tied to the Darkness, in return, the Hive must let themselves be infected with parasitic worms, and they must create as much destruction and death as possible. Also, they must consume all that is light. Edit: forgot to mention my point. We might have to face these Worm Gods (now only 3 exist).