The quote you cite in your question is spoken by the professor, and it starts the narrator thinking about whether his plan to kill the crapulent major is part of barbarism or civilization. He realizes that in the current day, it's hard to separate what is barbarism from what is civilization, as both sides commit atrocities. He also believes, as a Marxist, that "capitalism generates contradictions" (page 102), and he realizes that it is not only capitalism that generates contradictions; instead, the modern age is the story of right versus right, and there is no clear moral right or wrong.
This quote relates to the history of colonialism and the Vietnam War because each side in the conflict, Marxist and capitalist (represented by North Vietnam and South Vietnam, respectively), believed they were in the right (the Marxist North Vietnamese were also represented by the Viet Cong in South Vietnam). However, in order to pursue what they thought was right and moral, they committed barbarities so that neither side was wrong or right over time. The colonial powers in Vietnam--first the French, then the Japanese, then the French again, and finally the Americans--also believed in the moral superiority of their programs in the country but committed atrocities (including wholesale slaughter of Vietnamese people) in the service of what they thought was right. The quote is also about ethnocentrism because each culture and power thinks it is doing right; however, each side commits wrongs, and it requires a sense of cultural relativism to see the errors that all sides make.