How does Orwell describe his feelings about the British Empire and his role in it, both at the time he took part in the incident described and of writing the essay, after having had the opportunity to reflect upon these experiences?

Orwell certainly hates British imperialism at the time of the incident he describes in his essay. He writes,

For at that time I had already made up my mind that imperialism was an evil thing and the sooner I chucked up my job and got out of it the better.

He detests fulfilling the role of an imperial policeman, and he is disturbed and feels guilty about the way the locals jeer at Europeans and about the way prisoners are treated in Burma.
While he hates the empire, he is somewhat muddled and is perplexed about the whole picture of colonialism. He writes, "But I could get nothing into perspective." It is only after he looks back at the incident of shooting the elephant that he understands the rationale for crumbling empires (like the British in Burma) acting as they do. He realizes that he shoots the elephant only to avoid looking like a fool, and this corrupt motivation fuels the empire's feeble grasp on power. While he has always hated imperialism, he only comes to understand it after the incident he describes in the story takes place.

At the time Orwell (or, to be precise, the narrator) took part in the events described in "Shooting an Elephant," he describes himself as conflicted about the British Empire. On the one hand, he views the empire as fundamentally corrupt and morally wrong. On the other, he has little but contempt for the Burmese people themselves:

With one part of my mind I thought of the British Raj as an unbreakable tyranny, as something clamped down...upon the will of prostrate peoples; with another part I thought that the greatest joy in the world would be to drive a bayonet into a Buddhist priest's guts. 

Orwell goes on to say that this internal conflict is fairly typical of those in the imperial service, and this is underscored by his response to the incident with the elephant. Orwell does not really want to shoot the beast, which has calmed down after its rampage through the bazaar. But he feels that he has to, because the Burmese people expect it from him. He is expected to act with authority and with violence, and he realizes that, in his role as a representative of the Empire, he must live up to their expectations. So by the end of the story, he observes that he killed the elephant "solely to avoid looking a fool" in the eyes of the subjects of the British Empire. In retrospect, he recognizes that the essence of empire is violence.
http://orwell.ru/library/articles/elephant/english/e_eleph

At the time of the incident, Orwell describes his feelings about the British Empire in very negative terms. He talks about imperialism being an "evil thing," for example, and how running the empire is "dirty" work. He also refers to the British as "oppressors." To illustrate these points, he gives the example of Burmese prisoners, locked up in "wretched cages" and scarred from beatings.
Regarding his role in the British Empire, Orwell feels an "intolerable sense of guilt." Moreover, although he hates his job, he cannot simply give it up, which further adds to his sense of guilt and feelings of hatred towards the British Empire.
After reflecting on the incident, there is some change in Orwell's attitude. He is "glad" to have been on the side of the oppressors, if only because it gave him legal justification and pretext for killing the elephant. Moreover, from his tone in the final line, he seems amused as he wonders if anyone realized his true motive for killing the elephant: that he didn't want to look like a fool.