The statement is proven by this essay through the description of Orwell's dilemma as he faces the elephant that he feels he has to kill, in spite of his own feelings and ideas about what it is right that he should do. Orwell very clearly states that what he "should" do is to not kill the elephant outright, but to approach the elephant carefully and see if it really is a rogue elephant. However, what he feels as he watches the large crowd of Burmese people watching him and the elephant is pressure to do what they want him to do: to shoot the elephant. This, in the end, shows to be the greater force and motivator than Orwell's own thoughts about the situation. Note what he writes about this particular moment, and the epiphany that he experiences as he faces this conflict:
The people expected it of me and I had got to do it; I could feel their two thousand wills pressing me forward, irresistibly. And it was at this moment, as I stood there with the rifle in my hands, that I first grasped the hollowness, the futility of the white man's dominion in the East. Here was I, the white man with his gun, standing in front of the unarmed native crowd--seemingly the leading actor of the piece; but in reality I was only an absurd puppet pushed to and fro by the will of those yellow faces behind.
The irony of the scene is clear: Orwell would hardly describe himself as an Imperialist, but he is forced to act in a way that shows the "white man's magic" and mastery by killing the elephant, even though he doesn't want to. Even though he is "seemingly the leading actor of the piece," he actually finds himself to be increadibly weak and powerless. Through his actions, he is defined and thought of as an Imperialist, even though it is clear from reading the rest of the essay that Orwell feels at best extremely ambivalent about the how Imperial project, and at worst antagonistic towards Britain's right to have its own Empire. What defines him far more than these thoughts however, is how he acts, and as he clearly acts in a way that shows he is trying to prop up the rather fragile sense of Empire through acting in a way that is expected, he is defined more by these actions than his thoughts.