Orwell's "Shooting an Elephant" is concerned mainly with imperialism. Orwell was writing in the 1930s, at a time when Great Britain still controlled India as well as colonies in Southeast Asia, such as Burma, where this essay is set.
In the essay, Orwell argues that imperialism degrades both the rulers and the ruled, making them hate each other. He also reflects on how imperialism drives him, as a member of the ruling race, to do things he would rather not do, simply so as to not look weak in front of the "inferior" race.
While the theme of the essay is imperialism, there is some dispute among scholars as to whether Orwell is really anti-imperialist or if he condones it. Generally, pro-empire people think he was condemning it while critics of the empire feel he was condoning it.
He was trying to show how the colonizers and the colonized were divided and fixed into conforming with a general stereotype, in a certain sense.