The divisions in Nigeria are not all man/western based. Pre-colonial era southern Nigeria got its supplies of slaves for the triangular trade from inter tribal conflict. Northern Nigeria was a land of jihad where present day indigenous Christians are descended from those who resisted Islam. Colonial invasion brought an end to slave trading and eventually, as late as the 1920s, to slavery itself. Rather than divide people it united very disparate peoples in Pax Britannica. Christian missions bringing Western education and medicine were welcomed in the south but kept out of the north for many years by the British whose policy was indirect rule via Muslim emirs whose realms were consolidated at this time. Independent Nigeria lasted only six years before civil war erupted. The seeds of conflict were there from before the colonial area. It is not some supposed imposition of foreign ways that has lead to conflict.
Islam in Nigeria goes back to the Kanem empire in the 8th century. Co-incidentally it was in the NE of Nigeria, home of Boko Haram. Personal view of our empire there and everywhere else is that we were there for the money. Economics is the motivation. Worst thing we did in Nigeria, triangular trade. Best thing was stopping not only that trade but slavery itself but the latter was less than 100 years ago. But of five possible colonisers on the continent we were least worse option.