On their last day in Sighet, the Hungarian police crammed Wiesel and the other remaining Jews into the synagogue, where they were held for 24 hours. Symbolically, what is particularly upsetting about the treatment of this holy building?

I think the upsetting treatment this building is being given is that a synagogue is suppose to be a place where a Jewish assembly or congregation meets for religious worship and instruction and it is being used for a unsatisfactor reason of holding Jews while they wait to be deported. It is not being used for it's purpose.