In Chapter 5, why does Watson feel gratitude towards the murderer?

From the text:

Every time that I closed my eyes I saw before me the distorted baboon-like countenance of the murdered man. So sinister was the impression which that face had produced upon me that I found it difficult to feel anything but gratitude for him who had removed its owner from the world. If ever human features bespoke vice of the most malignant type, they were certainly those of Enoch J. Drebber, of Cleveland.

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