Who died at the conclusion of chapter Whom does katniss think killed tribute?

You're asking about the conclusion of Chapter 12, "Whom the Gods Love," in Suzanne Collins' *The Hunger Games*.

At the end of this chapter, Rue, the young girl from District 11, dies.

Katniss believes that the Careers killed Rue, specifically Marvel (the boy from District 1). She witnesses the aftermath of Rue's death and finds her body covered with flowers. This realization brings immense grief and anger to Katniss, which will propel her into a dangerous act of defiance in the following chapter.