How does Orwell show that imperialism harms the imperialist countries as well as their colonies?

Orwell's story "Shooting an Elephant" demonstrates that imperialism harms both the imperial countries and their colonies because the act of shooting the elephant is harmful to everyone in the story, and yet the narrator feels that there was no other course of action. By setting a precedent of oppression, the narrator has no choice but to continue to destroy his own self and to oppress others.
The narrator of "Shooting an Elephant" is a British police officer in Burma. He is called on to kill an elephant that has been terrorizing the local population by trampling local shops and huts. When the narrator arrives on the scene, he knows that the elephant is no longer dangerous and that he should not shoot it. However, as the crowd forms around him and watches his actions, he feels pressured into shooting it. The narrator does a terrible job of trying to kill the elephant, and ends up causing it a slow and painful death. He knows that he has cost the owner a lot of money by killing a valuable piece of property and that he was wrong to shoot it.
The message of the story goes beyond this one incident and is a commentary on the damage done to both imperialist countries and their colonies. The narrator recognizes that he is the oppressor and that he has caused great financial damage to the elephant's owner. However, he also recognizes that because he has bowed to the social pressure of needing to maintain authority, he has morally corrupted himself and allowed himself to make such a terrible decision "solely to avoid looking a fool" (Orwell). He knows that technically he was legally in the right to shoot the elephant "and afterwards I was very glad that the coolie had been killed; it put me legally in the right and it gave me a sufficient pretext for shooting the elephant" (Orwell). However, he also still knows that this is just a "pretext".
The narrator sums up his condition by saying,
I perceived in this moment that when the white man turns tyrant it is his own freedom that he destroys. He becomes a sort of hollow, posing dummy, the conventionalized figure of a sahib. For it is the condition of his rule that he shall spend his life in trying to impress the ‘natives’, and so in every crisis he has got to do what the ‘natives’ expect of him. He wears a mask, and his face grows to fit it (Orwell).
In other words, the narrator recognizes that when imperial countries colonize others they destroy their own humanity. After a while, they grow into the role and the only conceivable path they can take is to continue to oppress and dehumanize others.
http://orwell.ru/library/articles/elephant/english/e_eleph