29/07/2013 09:07 BST | Updated 28/09/2013 06:12 BST

When Does a Man Become a Monster?

In The Strange Case of Dr Jekyll and Mr Hyde, Robert Louis Stevenson brings to life one of the most compelling and original monsters of our time.

The obvious monster is Mr Hyde: the snarling, feral animalistic mass of murderous impulses. First, Hyde beats up a little girl. Later, he stamps an kindly white-haired old man to death - and enjoys it.

As the title of Stevenson's novella contrasts the characters of Jekyll and Hyde, so too does the reader. We meet the eminent Dr Jekyll, cowering in darkness, haunted, and feel sorry for him. But he is the real monster. Hyde's body is his body; Hyde's nature is his nature, and is his own creation. Dr Jekyll is a man who craves respectability at any price.

In order to preserve his respectability, and indulge his darkest desires, Dr Jekyll works on a compound that will transform him - separate his good and bad natures, and let it loose. It's an addiction. Though Jekyll is appalled, he always needs one more hit. He enjoys the transformation, enjoys the power, enjoys the feeling of keeping clean. With his alter-ego, Jekyll enjoys rolling in the dirt.

It's like the question: if a tree falls in the forest, does it make a sound? If we do mean, cruel, disgusting things, does it matter if no-one finds out? Various experiments have proved that in darkened rooms, more people will cheat for a financial gain. Under cover of darkness, when we think that we can't be seen, we lose our inhibitions. More crimes are committed under cover of darkness than in daylight.

Respectability and social pressures of the Victorian era are hard for us to understand today. We can easily understand that it's not okay to beat small children and kindly old men. But what's harder is to feel the social pressures that Stevenson was working in. In some ways, this morality tale is too sharply cut: none of us want to beat old men, or small children - hopefully. But we face temptations on a daily basis. Newspapers are full of public figures' moral failings: MPs fiddling expenses, Cameron bowing to pressure from the tobacco lobby, suicides of alleged-rape victims who weren't believed, where establishment interests were at stake. If your friend committed a crime, should you protect them? If you could get more money for your family, would you undermine a colleague's position at work?

In real life, the moral universe is rarely as clear-cut as respectable doctors vs murderers. It's about the subtle slide into the pit and the fact that often, there's something to enjoy on the way down. If there weren't something in it for us, we wouldn't go there in the first place.

Earlier drafts of the novel have a different slant: Jekyll's secret crime is not murder, it's homosexuality, and the psychological damage is not the consciousness of evil, but the damage of a torn nature. Society condemned homosexuality, and it had to be hidden. Ultimately, Stevenson abandoned this element - as too dangerous, too hot to handle. But it lies, still at the heart of the novel, making it hard to pin down whether Stevenson meant us to condemn social judgement, or condemn the person who thinks they can indulge animalistic urges as long as no one is looking.