When Katniss made the alliance with Rue, I don’t think she knew that i twas going to be really hard if Rue died, because she reminded Katniss so much of her little sister Prim. When they plan to set the distraction towards the career tributes, and the morning after Katniss cannot find Rue, she gets worried, and that’s when i know that i twas not a good thing because you knew that Rue was going to be killed and it would hurt Katniss. When Katniss hears a scream, she kind of knows that Rue is in danger, she follows the scream and sees Rue trapped in a net and then all of a sudden she has a spear go through her heart.
With Katniss gathering wildflowers and decorating Rue’s body and giving her a proper barial it’s a good thing because it can give some closure between her and how Rue reminds Katniss of her little sister. Katniss also thinks that it shows the Capitol that she is a real person and not just an entertainement piece for the people watching the hunger games.
Katniss knows that the game is on now and that she has to fight for Rue because in her last words Rue told Katniss she has to win the games for her and to sing to her as she dies.