You are assuming that the narrator is correct.
Look at the use of narration/voice over in the Coen ouvre and count the number of times the narrator has been, shall we say, reliable.

Lester crossed a line Peggy would never cross - he knowingly sent his girlfriend to die wearing his coat. Peggy has not knowingly put someone else in the firing line (at least, not in the same sense — she and Ed are equally naive about their situation until things flipped out in the cabin).