As sad as it may sound, I would almost expect this to be the way humanity ends up extinct should there be a few survivors after a world-wide war/massacre.
I think the author forced the gag a bit. I don't feel like either character had good enough motivation to commit a double homicide. Even if one of them had been successful, why would the survivor even want to stay with a double murderer? You could say their motivation and poor planning was due to the stress of a post-apocalyptic world, but I don't think that's a satisfactory explanation. I would have preferred to see the author throw out the gag ending and instead spend more time with the story and the awkward relationships between the characters.