Managing a large population was easier said than done, especially in terms of food. If they were to seek refuge, the food requirements would be astronomical. No matter how much was saved up, the reserves would be drained through rapidly.
Upon arriving at that conclusion, the Carcass Snail gave the order to its Mollusc Tribe, causing the Tribesmen to commit suicide by killing each other. It only spared the most evolved boy and girl, for they were the fruits of its efforts.
As long as they were alive, it could recreate the Mollusc Tribe to full strength in a couple of decades. They'll be superior to the existing Mollusc Tribe, so the Carcass Snail wasn't even concerned by their deaths.
The sole intention behind its actions was to conserve resources. Feeding two Tribesmen was nothing in comparison.
