why does seath describe the third time he sees the book thief as red

Death: "The last time I saw her was red." This is the fiery sky of a massive bombing raid. Death finds piled bodies stuck to the street and rhetorically asks if fate or misfortune glued them there. Sardonically, Death answers its own question: "Let's not be stupid. / It probably had more to do with the hurled bombs, thrown down by humans hiding in the clouds." Death finds the book thief kneeling among rubble, clutching a book. Death wants to console her, but "that is not allowed." Instead Death follows her; she drops the book and Death later takes it from a garbage truck. Death: "I would keep it and view it several thousand times over the years."

Death explains that these three colors -- red, white, black -- most resonate with its memories of Liesel, and draws them on the page as a dash of red, a circle of white, and a swastika for black. These are the colors and symbols of the Nazi flag; the implication is that Nazism is responsible for the deaths in these three episodes.