In the story's opening paragraph, the narrator describes one witness's experience of the crime, saying,

A maid servant living alone in a house not far from the river, had gone up-stairs to bed about eleven. Although a fog rolled over the city in the small hours, the early part of the night was cloudless, and the lane, which the maid’s window overlooked, was brilliantly lit by the full moon. It seems she was romantically given, for she sat down upon her box, which stood immediately under the window, and fell into a dream of musing. Never (she used to say, with streaming tears, when she narrated that experience), never had she felt more at peace with all men or thought more kindly of the world. And as she so sat she became aware of an aged and beautiful gentleman with white hair, drawing near along the lane; and advancing to meet him, another and very small gentleman, to whom at first she paid less attention.

Here, we have lots of description, visual imagery meant to help us to really see and experience the scene in our mind's eye, and this also helps to set the mood. The fog and the full moon make it sound sort of spooky and otherworldly as these natural phenomena are common indicators of menace and unpredictability. Further, the juxtaposition of the maid's sense of peacefulness with the horrible and vicious murder which she observes is jolting and helps to set the horror of the scene against a more contrasting backdrop, emphasizing the horror all the more. Additional description of the murder itself helps to convey this:

And then all of a sudden [Hyde] broke out in a great flame of anger, stamping with his foot, brandishing the cane, and carrying on (as the maid described it) like a madman. The old gentleman took a step back, with the air of one very much surprised and a trifle hurt; and at that Mr. Hyde broke out of all bounds and clubbed him to the earth. And next moment, with ape-like fury, he was trampling his victim under foot and hailing down a storm of blows, under which the bones were audibly shattered and the body jumped upon the roadway. At the horror of these sights and sounds, the maid fainted.

Hyde is disconcertingly described as possessing an "ape-like fury" with which he "clubbed" his victim to the ground. The bones, in a brilliantly vivid bit of auditory imagery, can be heard to snap and shatter as Hyde stomps on his victim's near-lifeless body. Therefore, the combination of lots of description with both visual and auditory imagery, along with juxtaposition, helps to convey the horrific violence and horror of the scene.

In Chapter 4, Hyde's capacity for evil is shown. His vicious nature becomes apparent with the violent murder of Carew. Hyde kills at random, with no apparent motive and little concern for getting caught, as he beats Carew to death in the middle of a...