The beasts of the Divine Sanctuary of the Beasts could never hope that the great sects would take advantage of that moment to retreat and go into isolation, now they would have to wait thousands of years or have to work hard to break the isolation of each sect and kill the elders of the Eternal Realm.
In order not to lose more beasts they intended to let the sects retreat, but they thought of killing 4 more elders from the Eternal Realm at least so that the divine plane could be considered destroyed.
So the beasts understood that Zhang Yi had already planned this and that is why he killed so many beasts just in the end, it made the beasts of Divine Sanctuary of the Beasts angrier, after all, Zhang Yi had come alone and changed the whole course of the war, now they had to kill Zhang Yi now to eliminate a problem in the future.