There are District-9 spoilers in this post. But seriously, go see this movie. These comments won't make much sense till you do.
So the film is about racism, tribalism, ghettoism, and the Other. Racism among humans seems to have dissipated once the prawn were known to exist. But that's how human tribes are. If the prawn left for good, humans would go back to segregating and killing one another (the word 'prawn' should be capitalized if it's used for a race, but I'll explain later why I don't think that's necessary).
I thought the film was overly intense, but no one else seemed to. Maybe they did and they didn't want to be called a sissy, but whatever, I'm more interested in figuring out what affected me. What I've come up with is that I'm really bothered by the idea of being pulled out of a shitty house, and put on your knees by people with guns. Whether you're a human, a prawn or Donald frigging Duck, it bothers me. Then shooting them in the head while they're on their knees? Yeah, that's too much.
It's not a video game movie, nor is it a sci-fi action flick, even D-9's most kickass fight scenes are too awkward for that. Even to the point that the commando bad guy who harasses Wikus, isn't killed by Wikus, but by a horde of prawn defending Wikus. Rather than a kickass moment, it's almost melancholy, a realization that he's past the tipping point, more prawn than human.
So here's the real braintease, as I see it. There are a load of unanswered questions (and talk of a sequel), but among that series of questions there are clues hiding.
D-9 left me wondering if the prawn species originally looked like the prawn form, or if it were a manifestation of the fluid. As if they were actually some third humanoid form, who had been exposed to the juice. What other purpose would there be for that fluid to exist?
Why would that prawn-fluid exist? Why would we humans create a fluid that turned another species, let's say dogs, into humans? To create an easily controlled sub-human, slave species, either warrior-slaves, or miner-slaves. That would explain why the tools and weapons are only activated by the species, or why other, larger equipment would run on the same prawn-fluid-fuel. The fluid is a DNA or virus and also a fuel, not a security measure, as we (or MNU) would perceive it to be. And the more fluid, the bigger equipment it can power, like the small ship. These different stages of the same fluid is addressed by CJ's 'chemistry set'.
Also, if they were slaves on a mining job, that would explain why they went to Africa. I think this is an obvious subtext, no need to get into it.
Bottom line, I think Christopher Johnson doesn't really look like that. I think he was exposed to the fluid, got sick with prawnism, and wasn't able to feed and care for his slaves. There are two levels of prawn: the workers, with no sense of direction, no ideas, no brains; and there are the three that we know of that have their minds intact, Christopher Johnson, CJ's child, and Wikus. Wikus was intelligent enough to build things, sneak out of D-10 and back in, all while remaining anonymous. This seems to point to the idea that prawn-form is an unnatural state. If a smart human becomes a prawn, he still thinks like a smart human (albeit a smart human trying to figure out how to survive in this form), but if a dog becomes a prawn, it behaves like a dog.
Which made me wonder why the slaves wouldn't rise up with the weapons, revolt against the owners? I don't think the slave-level prawn would revolt in their normal situation, they would just fill their purpose, in the same fashion as worker ants/bees/etc.
The 'unnatural state theory' would also explain why he has a technology to reverse the process. Just in case one of the higher-ups like himself were infected with prawnism.
That's my 2¢. Enjoy this sketch of Wikus van der Merwe.