Why does Jekyll become Hyde?

Because he didn't have Yahoo! Answers to give him stimulation and a nuisance cat to annoy him

Well, Dr. Jekyll was a good guy, and what he was searching to achieve was to get all of the evil out of himself, so that he could truly be a very good man, just like he wanted, and just like he hoped that everyone could do.

He took his potion... and he was partially successful. Most of any evil/bad/imperfect in him was banished. However, this created the alter-ego of Mr. Hyde, who was pure evil. Please remember that in the book, Hyde is not this huge bulbous muscular creature like they show in the movies. He was an old man, ugly and crotchety, who did unspeakable crimes.

The moral of the story is that a man (or woman) is the sum of his two parts, good and bad, and he must keep them in balance, or the natural equilibrium of Nature will be screwed. A man needs his evil to protect himself *and* help others, even if it may be based in evil. A man needs his good side to... do good things for his fellow man. They actually did a Star Trek (original series) episode just like this where Kirk was split into a wobbling wuss on one hand who was a useless commander, and on the other side, a ruthless person who would stop at nothing to satisfy his own ego. In the end, both with Jekyll and Kirk, it was the *good* side who realized that he need the bad, not the other way around -- this is important, because it demonstrate that the good side of a man is willing to accept the responsibility of using his "evil" strengths to do good.

So anyway, his defense, if he had a defense, would be just that -- Hyde was not Jekyll, and Jekyll was not Hyde. You did *not* murder for the greater good -- Hyde did it just to please himself. Just because he freaking wanted to. But Jekyll had nothing to do with Hyde, essentially. In a way, the Dr. Jekyll/Hyde were separate entities at the time.

Unfortunately, the prosecutor would probably say that it *was* Jekyll, just in his form of pure evil, which every man (or woman) carries within himself, something that needs to be used for good purposes, and kept in check.

Really good book, one of my favorites.

Ah, Jekyll was looking into the nature of man. More specifically, he was trying to understand why man does what he does. Jekyll as the medical man, the intellectual, the academic, could see the darker side of human nature, and tried to recognize positive aspects of it, increased strength, stamina, passion, but did not understand that these are inseparable from man's evil nature. He discovered too that man's evil nature is an inseparable part of man.

You will have to formulate your own case but this site will help with that.