Why did Stevenson need to kill lanyon off for purpose of plot?

We find all this out in the letter at the end. We find out Lanyon actually saw Hyde drink the potion and is afraid for his life. I think the killing of Lanyon adds a personal dimension to the horrific tale. Up until now, Hyde hasn't really done harm to people the main characters know of and respect. Lanyon is highly respected, rational, and values truth and goodness above all else. This death and Lanyon's final appeal brings the plot to a greater climax.