Why were the boys half-relieved and half-daunted by the beast?

The boys were made of the older ones and the younger ones.
Most of the boys were half-relieved, because rationally, they knew a beast really doesn't exist. However, they are stuck on a strange and unknown island, and the younger kids believed that it existed because Jack told them. Jack ruled by fear, and by telling them about a "beast" to keep them in fear.