When the boss sees the fly struggling to swim in the inkpot, he is at first sympathetic. The fly is described, seemingly from the boss's perspective, in the following passage:
At that moment the boss noticed that a fly had fallen into his broad inkpot, and was trying feebly but desperately to clamber out again. Help! Help! said those struggling legs. But the sides of the inkpot were wet and slippery; it fell back again and began to swim.
In response to this, the boss saves the fly with his pen and places it on some blotting paper. This immediate response indicates that the boss is sympathetic on some level.
After this, the boss begins to torment the fly, repeatedly covering the poor creature in ink, revealing his cruel side. He does this because he admires the fly's ability to clean itself and recover each time; however, on his fourth attempt, the fly doesn't survive. Importantly, the boss continues to torture the fly even after he realizes the creature is really struggling. The sadistic playfulness with which the boss submerges the fly suggests he can be callous and unfeeling:
He plunged his pen back into the ink ... and as the fly tried its wings down came a great heavy blot. What would it make of that! What indeed! The little beggar seemed absolutely cowed, stunned, and afraid to move because of what would happen next. But then, as if painfully, it dragged itself forward.
Not only does his treatment of the fly reveal his potential for cruelty, but it also reveals his tough and stoic nature. The fly continues to brush itself off and recover each time, which is something the boss clearly admires. This might suggest that the boss respects the kind of person who can recover quickly from setbacks without giving up. Given the time this story is set (in the 1920s, after World War I) this might indicate something about societal expectations and the fact that men were generally expected to remain uncomplaining and brave in the face of adversity.