It's a shame, because a more rational motivation is right there on screen, waiting to be used: The Nigerians collect alien weaponry, and like MNU, want to exploit Wikus to use them by any means. Instead, we're left to mull over witch doctors, and what could have been an important parallel isn't nearly as strong.
I thought it was perfectly obvious and needed no further justification. The Nigerian leader even says, after Wikus demonstrates that he can use the alien weapons, that he want to get an arm like that. The Nigerians use witch doctors and the MNU uses scientists – the respective "experts" in both cultures – but they're both just means to identical brutal and power-hungry ends.