The slaughter of the tribe people means the weakening of the entire race's strength, and the collective will of the tribe will become feeble. To the Emperor Sacred Beast, within the Beast Underworld, the decline of the tribe's fate will also reduce the boost to his own strength!
Bluntly put, the number of tribe members constitutes the foundation of the tribe's collective will.
Take the Human Race Group for instance, its collective will is so majestic and boundless, but if the Southern Domain did not exist, or if any of the Human Race's five realms were no more, then the collective will of the Human Race Group would weaken by a fifth instantaneously.
And the fate of just one side of the Earth is sufficient to support an emperor!
Today's Human Realm's Conquest of the Royal Beast Race is equivalent to digging at the very roots of the Royal Beast Race!
If the Emperor Sacred Beast was not slightly aware, not even ghosts would believe it!