The first horrifying sight in Night comes in the first chapter, where Moshe told of horrible stories. According to Wiesel, Moshe said:
"Babies were thrown into the air and the machine gunners used them as targets. This was in the forest of Galicia, near Kolomaye."
After this, Wiesel stated that Moshe went through long nights and days going from house to house to warn Jews. Moreover, Moshe and Wiesel were friends. They were close enough for Wiesel to see a definitive change in Moshe.
"Moshe had changed. There was no longer any joy in his eyes. He no long sang. He no longer talked to me of God or of the cabbala, but only what he had seen."
Even if the people of the town and Weisel politely brushed him off, they knew something had happened. This was their first brush with evil. Later they would experience it firsthand, but the first horrifying sight was a changed man, Moshe.
A good argument can be made that everything Eliezer witnessed in Night was horrifying. For example, when he hears from Moshe what the Nazis did to infants, using them for target practice, to the evacuation of Sighet and the people being hurried along by abusive SS officers, all of these could be horrifying sights that he at first disbelieves. However, I think that Eliezer arriving in Auschwitz and seeing the full extent of the Nazi machinery of death was something that caused an immediate sense of disbelief. For Eliezer, someone who believed in the restorative powers of the divine, to confront the horror of the Nazis in Auschwitz caused disbelief. When he was separated from his mother and sisters, Eliezer bore witness to horrifying sights which triggered an initial disbelief. In the poem "Never Shall I Forget," Eliezer writes of "that smoke" and "the small faces of the children whose bodies I saw transformed into smoke under a silent sky." Eliezer is unable to believe that a redemptive divine being would so willingly sacrifice children to death. Eliezer notes that he is unable to comprehend "the moments that murdered my God and my soul and turned my dreams to ashes." While he is unable to forget these sights, it makes sense that his initial impression of them would be to greet them with a sense of disbelief. Over time, the memories hardened his soul to a point where the horror of the Nazis was fully understood and grasped. For Eliezer, standing witness to the Nazi cruelty that is Auschwitz represents the first horrifying sight towards which he could only voice disbelief.