Why are the children fearful of the professor?

In the story, the children are initially afraid of the professor because of his reclusive nature. He keeps to himself and seemingly admits no friends into his company. Because he is an unknown entity, no one can rightly say what he's like. Certainly, no one can predict how he will react in any given situation.
The professor's forbidding appearance also makes him a figure of fear in the children's neighborhood: he is tall, bent, bearded, and possessing of a most forbidding stare.
Although the children eventually use the backyard of the professor's store for their Egypt Game, he continues to remain a mysterious figure to them. The professor's reputation is further eroded when he becomes the main person of interest after a little girl is killed in the neighborhood. Almost overnight, the entire neighborhood becomes inordinately suspicious of the professor. A rumor begins the turn of public opinion against the elderly man. Accordingly, someone had seen two police officers enter the professor's antique shop the day after the little girl disappeared.
This is enough for some people to think that the professor must be guilty of the crime. Soon, someone throws a brick through the professor's store window, and Mr. Schmitt organizes a campaign to kick the professor out of the neighborhood.
Notably, the Egypt Game children believe he is innocent, only because it would be terrible to imagine playing in the backyard of a murderer. So, more than anything, the fear of the professor is inspired by the fact that he is a reclusive figure and an unknown variable in the social fabric of the neighborhood.