Dr. Griggs could not find anything physically wrong with the girls so he determined they were bewitched. During this time period, people believed sickness was caused either by natural or supernatural causes. So, when Dr. Griggs' medical books had no explanation for what the girls were experiencing, he assumed it was witchcraft.
Napoleon's military background shaped his perspective by making him believe war was one of the only ways to solve issues. He was a military genius, however because of this he got into a lot of trouble.