Death foreshadows Rudy's death to further the characterization of Death itself and to heighten the poignancy of Rudy's remaining life.
After jumping into the Amper River, Rudy proudly holds up the book he has retrieved, teasing Liesel that his brave actions deserve a kiss. Since the air is a "nauseating cold" and the water is dangerously icy, his actions place him at risk of hypothermia. Death predicts that readers will assume that Rudy's submersion into the frigid water led to his demise and therefore foreshadows Rudy's actual death, which occurs "a little under two years later."
Death feels that taking Rudy, who is so full of life in his youth, is "robbery." This characterizes Death as rather tender, which is an unexpected and atypical association. Death describes Rudy as a boy with "so much life, so much to live for," and mourns the loss of that potential. Hence, Death reminds readers that "even death has a heart."
By foreshadowing Rudy's death, Death also asks readers to more closely examine the way Rudy lives his final two years. It increases the anticipation of Rudy's departure from life as readers are not aware of exactly how he will eventually meet his end. This scene also presents a juxtaposition of Liesel's own actions. When Rudy is alive, he begs Liesel for a kiss as a thanks for the bravery he demonstrates in the river. It isn't until he is dead that she "kiss[es] his dusty, bomb-hit lips."
This foreshadowing increases the irony in this juxtaposition of action and inaction. Rudy is only able to effectively sway Liesel's heart toward action when he is dead and it is too late.