Hissing was making a whistling sound through the teeth. It may have been out of dismay, grief, or surprise. (See: [[rc://en/ta/man/translate/translate-symaction]])
The abstract noun "horror" can be translated using the adjective "dreadful." This can be stated in active form. Alternate translation: "you have become dreadful" (See: [[rc://en/ta/man/translate/figs-abstractnouns]])