When Bruce Wayne was in his study in Wayne Manor he was wondering what would be an effective way to fight criminals. And just then, like an omen, a bat flies through the window. He then realized that a bat is a terrifying creature, and that if he dressed up like a bat, he could strike terror into criminal hearts. During his studies, he learned that criminals were, for the most part, a superstitious, cowardly lot. So he knew he could use that to his advantage.