Yeah, the structure is weird - I think because of the obvious allegorical nature of the set-up, the writer-director went out of his way to frustrate any clear allegorical reading. That in itself could make for an interesting analysis though, in terms of why the allegory was deformed in certain ways and not others. It's crucial to the plot that it ends up a three-way battle between whites, blacks and aliens. But the blacks are Nigerian mobsters rather than black South Africans, who are curiously absent from any major role. The sympathies are obviously with the aliens, and against the whites/government, though it relies on the device of a white being transformed into an alien.
Purely in terms of tension and action, though, it's an awesome movie.
Cheers, Mike