I don't know any Nigerians, but a protege of mine is a first gen Nigerian-American who was admitted to an Ivy League medical school out of high school. To say she is exceptional is an understatement. Not that I think any one person is representative of a group, but I can certainly attest to the fact that not all Nigerians are scammers, drug runners, or pimps and whores.
Well, that's good to hear. I'd guess that opportunity plays a role. In Ho Chi Minh City there were a lot of Nigerians who held business visas and were infamous for fighting with each other in the streets. They were also known to sell drugs and to service female sex-tourists. The government got sick of it and then came down really hard on all foreigners and deported quite a lot of the Nigerians and for about 6 months it was really tough to get anything but a tourist visa, meaning that a lot of foreigners were forced to go without proper documentation for a period of time, some for more than 6 months. I get why border controls exist and the arguments against illegal immigrants, but it can be very, very difficult to operate 100% aboveboard, especially when the bureaucracy is so bloated, or alternatively, in an environment where bribery is the norm (not that I am condoning undocumented work).