The British colonial District Commissioner seems to embrace uncritically his responsibility to pacify and .the indigenous African people. The idea of "peace" is important to him, even as he understands that "pacification" must likely be achieved through violence. He is an administrator, however, not a military officer so he can separate his role from the armed conflicts. Understanding himself as a representative of the queen, he also believes that the British have the right to replace the native rulers and their system of justice—including war and revenge—with British bureaucrats and courts.

The commissioner is proud that the British "have brought a peaceful administration," and he claims that the goal is to make the people "happy.” Castigating them for violence, he reminds them of proper conduct in "the dominion of our queen." With "our," he makes it seem that she is the Igbos queen as well.

As the book draws to a close, he ponders their national mission and his role in it, considering the book he could write: "The Pacification of the Primitive Tribes of the Lower Niger."

I don't know of a place in the novel that directly answers this question, but there are places where the District Commissioner is mentioned that help to explain.

Initially, the first white men in Umuofia were the missionaries.  Afterwards, others began to come in.  In chapter 20, it mentions the DC judging cases in court, chapter 21 mentions the DC being a messenger of the Church of England, in chapter 23 we see the DC trying some men from Umuofia in court,  it also talks about the children of Umuofia having to go to the white man's school.  

The final chapter of the book gives the best indication of why the DC and the British were in Africa.  The title of the DC's book is "The Pacification of the Primitive Tribes of the Lower Niger".  The British, as they were colonizing, actually believed they were doing the tribes a favor.  They felt they were teaching them to be civilized and live in 'right' ways.