They give humans a lot of reasons to dislike them and be offended by them. But the challenge is to rise above those urges and abstain from falling prey to racist rationalization.
But the same standards, it seems, are not being applied to the Nigerian gangsters in the film.
Not really. The only reason to be repelled by the prawns is really pure simple revulsion at their appearance; they're not given any twisted agendas or at any point in the movie painted as villains.