Holmes says that evil doctors are the most destructive.  About Dr. Roylott he notes, “Subtle enough and horrible enough. When a doctor does go wrong he is the first of criminals” (enotes text p. 17)  He describes him as having nerve and knowledge, meaning because he is a doctor he knows just what to do to kill someone.  Since he is evil, he has the desire to kill someone in a terrible way.

This man strikes even deeper, but I think, Watson, that we shall be able to strike deeper still. (p. 18)

Holmes is even temporarily stymied by the doctor’s plan with the gypsy red herring, and acknowledges that the doctor almost bested him.

The idea of using a form of poison which could not possibly be discovered by any chemical test was just such a one as would occur to a clever and ruthless man who had had an Eastern training. (p. 20)

Holmes is very aware of the doctor’s cunning and deceit.  He knows that they are in danger, but he also knows that as smart as the doctor is, he is even smarter.  The various exotic animals that run free unnerve Watson, but Holmes is nonplussed.

When Holmes whispers to Watson that “the least sound would be fatal to our plans” (p. 18), he is aware that the doctor will notice if they do anything unusual.  Holmes knows the doctor will try to kill them by sending the snake into the room.  When the snake comes in, Holmes descends on it quickly and pummels it with his cane.  They then follow the snake back to Dr. Roylott’s room and find him dead with the “speckled band” around his neck, the snake.

“It is a swamp adder!” cried Holmes; “the deadliest snake in India. He has died within ten seconds of being bitten. Violence does, in truth, recoil upon the violent, and the schemer falls into the pit which he digs for another” (p. 18-19).

Holmes notes that by attacking the snake he caused the death, and he notes that it doesn’t bother him much since he was an evil man.

“Some of the blows of my cane came home and roused its snakish temper, so that it flew upon the first person it saw. In this way I am no doubt indirectly responsible for Dr. Grimesby Roylott's death, and I cannot say that it is likely to weigh very heavily upon my conscience.” (p. 20)