Why does Doctor Watson write that the cry of pain from the hound had ended our fears? (Chapter 14)

The cry of pain solidified the belief that the Hound was not some kind of immortal, mystical evil..... it could be hurt, and it could be killed.

But that cry of pain from the hound had blown all our fears to the winds. If he was vulnerable he was mortal, and if we could wound him we could kill him.