On the face of it, it seems that Duncan's killing of his friend Carl is completely senseless. Here we have a young man from a good family who'd never previously shown that he was capable of committing such a serious crime. And yet, for some reason, he's shot his friend in what appears to be a cold-blooded killing.
But if we dig a little deeper, a more complex picture emerges. And this is where the symbol of the gun comes in. Duncan lives in a country with a catastrophically high level of violent crime, a country with one of the highest murder rates in the world. Unsurprisingly, this is also a country where guns are freely available.
Taking all that into consideration, one can argue that the gun is a symbol of the violence that pervades South African society. It can also be said to represent a toxic environment in which complex emotions, instead of being expressed in words and civilized deeds, are expressed instead through acts of murderous violence.
This violence is so pervasive that even those who, in their complacency, imagine that they're somehow immune to it succumb to its deadly temptations. Even though apartheid may be long gone, its legacy of violence remains, adversely affecting the descendants of its architects.