This is a tale drawn in shadows and draped with masks befitting the occupation of Charles Augustus Milverton, the black trade of extortion.
In answer to Watson's inquiry as to who Milverton was, Holmes likens the experience of being in his presence as "standing before the serpents in the Zoo and see the slithery, gliding venomous creatures with their deadly eyes and wicked flattened faces." In the second illustration, which depicts the first attempt by Holmes to take the law into his own hands, Milverton is pictured as described. As if at a Zoo, Holmes and Watson stand apart from Milverton whose serpentine nature is enhanced by his slithering against the wall and the shadow which creates a second skin for this scaly creature. Holmes' own shadow falls away from the light and ends to the left of this not so cornered enemy of the people of power and wealth.
It is a darkly drawn scene that finds Holmes and Watson in masks and shadows inside the mansion of Milverton. Watson hangs back as Holmes listens at the door. His ethical conscience may have been satisfied that their actions were morally justifiable, but his not so adventurous heart leaves him flat footed at the instant burgling the home to which Holmes so intently leans into as he listens at the door.
Mardi Gras-like pieces of silk adorn their faces, but they are but the first masked vigilantes to enter the mansion this night. The unnamed aristocrat who will soon end Milverton's life stands before him in her silk gauze as he confidently gazes up at her from his chair. Her hand is already parting the robe that contains the revolver that will be emptied into his chest, but there is no indication that Milverton suspects that one, let alone three, threats to his enterprise lurk in the shadows of his room.
The climactic frame shows Milverton, who has at least five rounds in his chest already, staggering up from the floor with a clean shirt front seeking the coup de gras which is administered forthwith. Mask removed, the female stands majestically above Milverton, who again looks up at her, but this time as she gains control of the meeting and his fate. Paget has dropped the "Pickwick" look of the earlier drawings, and has drawn a reptilian torso and legs on this "worst Man in London."
The two burglars retire to Baker street for soul searching and cover, conferring on the unknowing Lestrade a less than discrete confession of their participation in the affair, before venturing out to satisfy their curiosity as to who the midnight assassin of Milverton was. In a scene much like standing at a zoo and observing its occupants, Holmes leans forward into the glassed fronted widow bearing the image of she who struck with vengeance in the dark, while Watson stands a pace back .
Perhaps he is contemplating the joke his friend had made to Lestrade when he fingered Watson as the intruder.
Perhaps he is wondering about the ethical trade Holmes has made in insuring the happiness of an aristocratic woman by exploiting the emotions of the lower class made he compromised to find the floor plans of the Milverton house.
More likely he is simply awaiting the further instructions of his accomplice in crime before turning down the gas lit Victorian street Paget has drawn as the exit piece.