How does the idea of fate vs. free will appear in "Flight"?
A great place to look for evidence of this would be in the way that Pepe describes his fight with the man that he eventually ends up stabbing. He seems to refuse to try and take responsibility for his actions through the way that he describes what happened. Note the following description:
the man started toward me and then the knife—it went almost by itself. It flew, it darted before I knew it.
The way that Pepe describes the event makes it evident that he sees something fateful in it rather than it being a conscious decision that he made as the master of his own destiny. At the same time, we could also look at the way in which, when Pepe leaves his mother, and Emilio asks Rosy if he is dead, Rosy responds very tellingly by saying "He is not dead... not yet." This suggests that fate is a more powerful force than free will in the way that Rosy seems to be able to implicitly acknowledge that fate has decreed Pepe will die, and it is just a matter of time of when.