Killing retreating soldiers.... doesnt exactly seem like the honorable thing to do... where there any general codes about this? I just have this image of an invading army slaughtering people down to the last man even if its obvious they've already won.
In point of fact, most casualties in battles come not from the combat phase, but from the pursuit afterwards. More can always be said on the matter; while we wait for fresh material, u/Hergrim has looked at when casualties were sustained in this previous post and also in this thread.