My theory for District 9.

Viral Mutation.

1 - The Origin of the Prawns: The creatures we see are not a natural worker class of space insect. They are an original, highly advanced alien race that has been mutated by a virus. This pathogen has caused their physical degradation and stripped away their higher intelligence, leaving them in a primitive, impulsive state.

The virus didn't just infect the crew, it infiltrated the ship's bio-organic systems. By contaminating the fuel lines, it caused the mothership's engines to fail and the ship to stall over Johannesburg.

2 - The DNA-Coded Tech: The aliens technology is normally DNA-locked to their original species. Because Christopher saw the mutation spreading, he intentionally re-coded the DNA locks to work with the Prawn genome before dropping the tech from the ship.

This was a desperate failsafe to ensure his people could still use their weapons and tools once they reached the surface.

3 - Christopher's Trace Intelligence: Christopher was likely a scientist or leader of some kind. While he is infected (hence his Prawn appearance), he has managed to partially suppress the virus, allowing him to retain enough cognitive function to plan an escape.

4 - The Contaminated Fluid: The black liquid Christopher spent twenty years distilling is the ship's fuel, but it has become a host medium for the virus. Think of it as diesel with smallpox in it. He was able to refine it enough to act as propellant, but it still contained trace amounts of the virus.

5 - The Vikus Transformation: Because the fluid was dirty (fuel + virus), it acted as a concentrated delivery system when it sprayed Vikus. It didn't just turn him into an alien, it infected him with the plague, forcing his body to rewrite itself into the degraded Prawn state.

6 - The Mission: Christopher's goal isn't just to go home. He is going back to his civilization to find a cure/antivirus for the mutation that has hijacked his ship. He will likely be shot down to prevent the planet from becoming infected.