How does Stevenson make Jekyll and Hyde such a powerful horror story?

Stevenson was able to depict such horror and evil in his character of Mr. Hyde, that the reader attributes him with nothing but evil deeds. Yes, a girl gets trampled...... and much later, an elderly man is killed, but what captures the reader's attention throughout? Mr. Hyde challenged all propriety, he dabbled in everything seen as debauchery, and he enjoyed it, and that is exactly the reason Jekyll created his alter ego to begin with. He is described as evil and deformed but we never get a description..... that's what holds the reader's attention and makes the story so powerful..... our own imaginations.