When Cato returns with his allies, he descends into a fit of rage and snaps the neck of the District 3 male, killing him instantly.

Who got their neck snapped in the Hunger Games?

Death. When the Careers returned, Cato, in a fit of rage, confronted the District 3 male and stripped him of his spear, the only means of protection he had, and promptly snapped his neck as Katniss watched from behind cover nearby. He placed 9th out of 24.

Why does Katniss kill Cato?

When Katniss kills Cato to end his suffering she shows how truly compassionate she can be. Often it is easy to be compassionate to someone who is kind, but Cato was deadly and had tried to kill Peeta earlier that day.