If there is a villian in the actual games, it is Cato. The cocky muscular tribute is an unrelenting killer willing to snap the neck of an ally if a plan falls apart. Towards the end of the film, the stage is set for a showdown between him and the Katniss/Peeta alliance. As the numbers in the game narrow, a District 11 tribute named Thresh saves Katniss' life when he realizes how much she did to protect Rue throughout the games.
After he saves Katniss, though, Thresh is ultimately murdered. In the book, the assumption is that Cato has killed him. But in the movie, the suggestion is that Thresh has died at the hands of a group of dogs that the game makers have called into battle to help kill some of the remaining tributes. This difference - which may seem small-- is actually quite important because Cato's murder of Thresh in the book helped set the stage for a more intense final showdown between the surviving tributes.