Hyde, as his name indicates, represents the fleshy (sexual) aspect of man which the Victorians felt the need to "hide" — as Utterson once punned on his name: "Well, if he is Mr. Hyde, I will be Mr. Seek."
Hyde actually comes to represent the embodiment of pure evil merely for the sake of evil. When he is first extracted and in our first encounter with him, he is seen running over a young girl, simply trampling on her. He does not do this out of spite — or intentionally; it is simply an amoral act. He does make reparations. But even in this first encounter, he raises a fear, an antagonism, and a deep loathing in other people. The reaction of others to him is one of horror, partly because while looking at him, others feel a deep desire to strike out at him and kill him. In other words, his mere physical appearance brings out the very worst evil in other people.
Since Hyde represents the purely evil in man (or in Dr. Jekyll), he is, therefore, symbolically represented as being much smaller than Dr. Jekyll — Jekyll's clothes are far too large for him — and Hyde is also many years younger than Jekyll, symbolically suggesting that the evil side of Jekyll did not develop until years after he was born.
Hyde also creates terror; the servants are extremely frightened of him. When they think he is around the house, the servants cringe in horror, and some go into hysterics.
As the novel progresses, Hyde's evil becomes more and more pronounced. He bludgeons Sir Danvers Carew to death for absolutely no reason other than the fact that Sir Danvers appeared to be a good and kindly man — and pure evil detests pure goodness.
Since Hyde represents the evil or perverse side of Jekyll, and since Jekyll does, vicariously, enjoy the degradations which Hyde commits, Hyde gradually begins to take the ascendancy over the good Dr. Jekyll.
A conflict between them erupts, as though the older Dr. Jekyll is a father to the errant and prodigal son. He wants to punish this son, but at the same time, he recognizes that Hyde is an intimate part of himself. Ultimately, when Jekyll commits suicide in order to get rid of Hyde (suicide is an evil act in the eyes of the church), this allows Hyde to become the dominant evil figure, and the dying Jekyll becomes Hyde in the final death throes.