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Rudyard Kipling wrote "The White Man's Burden" as a white supremacist justification for the horrific effects (most notably, slavery) of colonization on people of color around the world during--and following--the era of European conquest and expansion. Kipling uses the concept of "social Darwinism" as a white supremacist theoretical framework for his poem.
Social Darwinism is the belief that certain groups of people are inherently better suited for survival and more evolutionarily "fit" to thrive than other groups of human beings. This belief was specifically used by white supremacists to justify colonization and slavery, as white supremacists insisted that people of color were less evolutionarily "fit" to function within civilization and therefore needed to be controlled, and outright owned, by colonial white powers for their own benefit. Of course, there is no actual scientific basis for social Darwinism, and many scientists were--and still are--horrified that evolutionary theory would be used to encourage and enforce the exploitation of an entire population.
In "The White Man's Burden", Kipling states that white men have a "burden" to colonize the non-white world and, essentially, "save" people of color from themselves by enslaving and colonizing them. Kipling believes that, through social Darwinism, white people are inherently more fit to survive and thrive in civilization and that people of color should be thankful to be controlled and enslaved. However, Kipling believed that people of color were basically too savage to understand that they needed to be saved, and thus required white people to be paternalistic figures in their lives. As such, in the poem, Kipling sees the "white man's burden" as a thankless position that white people must endure.
The blatant and horrifying rhetoric that is evident in this poem aligns with the narrative of white supremacy, which led to millions of indigenous people being massacred in widespread genocide and millions of Africans being enslaved. Eventually, the ideology inherent in "The White Man's Burden" would result in millions of black and brown people being stripped of their political, social, economic, and individual autonomy.
The entire poem is certainly an example of ethnocentrism; Kipling's belief that white people have a burden to impose white civilization and culture upon people of color is a clear example of the idea that one culture can be superior to another. The "half-devil" and "half-child" people that Kipling refers to are just other human beings who are not white and do not participate in white culture. The people he is referring to have their own political, cultural, social, and economic systems, as well as their own unique ways of understanding the world through their cultures, traditions, beliefs, values, and stories. Given that white, colonizing nations were responsible for a staggering amount of worldwide deaths through genocide, ecocide, and exploitation, it is ironic that Kipling would frame people of color as the "uncivilized" ones.
Even the notion that living within a civilization is a defining factor regarding one's intelligence or strength of character is a white supremacist concept. "Civilization," and the concentration of power and hierarchy through the creation of civilization, has been the root of many horrific oppressions that exist currently in our own world. For example, in our modern world, people living outside of what is generally considered to be "civilization," or large urban centers, are significantly less responsible for the current ecocide of the planet and the massive exploitation of natural resources that occurs across the globe.