Hernan Cortes was afraid of the Aztecs because they were the strongest group of people they had come to so far. Cortes was power hungry and greedy, he wanted what the Aztecs had, even though fighting them would be difficult. Hernan Cortes’ bottom line was wealth and control, always.
Your answer is... Hernan Cortes was horrified because the Aztecs were the most advanced, strongest group he had yet to come across when colonizing that area.