What is the purpose of complications in a novel's plot? How do complications relate to the conflict?

Hich excerpt from "the cask of amontillado" best infers what will be fortunato's downfall? the gait of my friend was unsteady, and the bells upon his cap jingled as he strode he had a weak point --this fortunato --although in other regards he was a man to be respected and even feared. he prided himself on his connoisseurship in wine. "nitre," i replied. "how long have you had that cough? " "my friend, no; i will not impose upon your good nature. i perceive you have an engagement. luchresi--"

How does this passage contribute to the overall meaning of the story

Consider tim o’brien’s story "ambush." what differences do you see between how o’brien dealt with killing an enemy soldier in the moment of battle compared to looking back on the moment later in life? what might this contrast reveal about the effects of war?