Question

How does calling the hunt for tutsis a sport event convey the tone of the narrative

Answer 1

It helps convey the tone by stating that the Hutu's saw the Tutsis as below them, and therefore taking their lives would be equivalent to taking the life of an animal, as many hunters do for sport. Basically, they are comparing the Tutsi people to animals and saying that their lives are less valuable.

Answer 2

Calling it a sport makes it seem less unpleasant, more acceptable.  If you called it a massacre, it would sound horrible and violent.