Bruno's father, the Commandant of "Out-With" is seen by his little boy as elegant and dashing in his dramatic black SS uniform. From his actions, it becomes clear that Father does indeed become the person the uniform demands. He makes changes that makes Out-With more efficient. And since Out-With is what young Bruno calls Aushwitz, more effiencient means more effective in carrying out the offical plan of the legally elected Nazi government to obliterate the Jews of Germany, and then the Jews of all conquered countries.
By comparison, when Bruno puts on the filthy garb of a concentration camp inmate, the striped pajamas, he remains Bruno, a curious, self absorbed little boy who wants to go home in time for a dinner of roast beast and remains oblivious to the horrible reality of what is happening around him.