Why is President Snow there to see Katniss?

Basically, Snow visits Katniss in order to threaten her. In Chapter Two of the novel, President Snow comes to inform Katniss that there are rumors of uprisings caused by her defiance in the arena. He informs her that it is her responsibility to quell the discord in Panem by convincing the people that her actions were not based on defiance, but rather, that they were based on love (love for Peeta). Snow goes onto threaten her that if the uprisings occur, he will kill Gale Hawthorne.