Was Orwell a victim or oppressor?

In a sense, he's both. He's an oppressor in that he's a colonial police officer, a cog in the machine of British imperialism. His job entails the maintenance of order and stability in this far-flung corner of the Empire. If any of the indigenous people should step out of line and challenge their colonial overlords, he'll be required to mete out condign punishment to the transgressors.
At the same time, Orwell is a victim of the very system he serves. In "Shooting an Elephant," his duty as a colonial police officer forces him to act against his better instincts. He doesn't want to shoot the elephant, but he feels an overwhelming burden of expectation bearing down on his shoulders, both from his superiors and from the indigenous Burmese, who expect him to deal with such fraught situations in an orderly manner.