TWO IMPORTANT CONFIRMATIONS!
1. The first time you walk down the river, and there are enemies on the hover-scooters, if you shoot them with a tranq and they fall down and explode, it does NOT count as a kill.
2. You can determine if you have killed an enemy when you are "fighting" The Sorrow. He specifically says "These are the creatures you have killed". The Fury, The End, The Pain and The Fear will show up regardless of you killing them or defeating them by stamina draining. If while walking down the river you see anyone other than The Fury, The End, The Pain or The Fear, you've killed an enemy and you will not get the Peace Walker achievement.