I think it was mid seventies I read this, not sure if it was a novellete or a longish short story.
It begins in a very strict boot camp for Galactic Police, the trainees are the toughest of the tough from around the galaxy and endure a really harsh winnowing process with a massive dropout and death rate
The big threat against galaxy peace and harmony is a notorious crime syndicate with a brutal masked leader.
Protagonist and his roommate get through to graduation, at the ceremony some are assigned animals to work with, one graduate is given a genetic modified camel that promptly bites his entire arm off so he fails at the last hurdle.
The two roomies are assigned to a scout ship together, after three or four routine incidents (I'm not sure about this bit) they make some kind of error and are called in for discipline. (Still not sure) but they somehow come up directly against evil masked leader alone in a room.
Roommate swiftly shoots him and they are amazed when they unmask the dead man to learn by ID papers that it is the Space Police supreme commander (I think this person is also always incognito to prevent retaliation)
Roommate swiftly dons the clothes and mask to take over the role and the power He turns around but protagonist has weighed up the situation and fled in scout ship.
The story ends with protagonist in hiding in a city and forever changing his guise and identity, he knows he is being hunted now by both the police and the criminals because he's the one man who knows the truth