Darius is Katniss' nice, good friend, and he pays a terrible price for being, well, nice and good. He's a District 12 Peacekeeper who doesn't really know his place. When he steps in to try to protect Gale from being beaten by the new Head Peacekeeper, he's soundly beaten for his pains and carried off for an unknown punishment.
Darius is being made an example of, it turns out. When we see him again in the Capitol, right before the Quarter Quell, he's been stripped of his voice and made into a mute servant called an Avox. He actually has to serve Katniss and her friends. Katniss thinks what happened to him is meant to be a warning to her, and she takes it seriously.