Mr. Hyde is supposed to be completely evil. He goes out on the town to enjoy himself by reveling in activities which are supposedly immoral, wicked, and criminal. Dr.Jekyll presumably enjoys all this wicked behavior along with Hyde. Yet because of the prudery and inhibitions and censorship of Victorian Britain, Robert Louis Stevenson was unable and unwilling to describe any of the illicit pleasures indulged in by Mr. Hyde and vicariously by Dr. Jekyll. It is significant that when the story finally comes to the chapter in which Dr. Jekyll is supposedly confessing everything in a written statement titled "Henry Jekyll's Full Statement of the Case," he writes:
Into the details of the infamy at which I thus connived (for even now I can scarce grant that I committed it) I have no design of entering; I mean but to point out the warnings and the successive steps with which my chastisement approached.
The entire short novel is full of hints, veiled suggestions, circumlocutions, and stuffy language which make it difficult and sometimes tedious reading. Jekyll says he has no "design"--that is, no intention--of confessing anything at all about what he and Mr. Hyde were up to during their nights on the town. In modern American vernacular this might be called a "cop-out." All we know for sure is that Hyde murdered a man in a fit of rage and that on one dark morning he knocked a little girl down and trampled on her for no reason except that she was in his way.
Stevenson must have felt that his story was crying for explicit descriptions of human wickedness, and yet he must have realized that nobody would publish it if he described Hyde partaking of some of the possible sins that were abundantly available in Victorian London. Prostitution flourished. The use of opium was apparently widespread. When Stevenson makes a big issue out of Hyde's trampling on a little girl, this is probably a cryptic way of suggesting that Hyde committed all kinds of sex crimes, including crimes against children. Hyde was capable of the most sadistic and perverted deeds that any human being is capable of doing--and yet his alter ego Dr. Jekyll, because of Victorian sensibilities and Victorian hypocrisy, is forced to write:
Into the details of the infamy at which I thus connived (for even now I can scarce grant that I committed it) I have no design of entering...
To the question, "Why did Mr. Hyde run over the girl?" I would answer that Stevenson had to show him doing at least something wicked and he couldn't describe any of the really wicked things Hyde was probably doing. The idea of a man knocking a little girl down and deliberately trampling over her body is a bit ridiculous. Stevenson just couldn't think of anything better--at least anything printable.