Final answer:
Dr. Lanyon is shocked and horrified by witnessing Mr. Hyde's transformation into Dr. Jekyll, which goes against his scientific beliefs and leads to his death.
Step-by-step explanation:
Dr. Lanyon's reaction to Mr. Hyde's transformation into Dr. Jekyll is one of incredible shock, horror, and disbelief. The sight of the transformation profoundly disturbs Dr. Lanyon, shaking the foundation of his scientific and rational understanding of the world.
Jekyll's explanation, which provides evidence of the dual nature of humanity, is in stark contrast to Lanyon's conventional and scientific worldview. Dr. Lanyon is a man of science, and the supernatural nature of Jekyll's transformation challenges everything he knows to be true, resulting in his eventual death from the shock.