That not kill any monsters achievement just adds more to the idea that the monsters in the silent hill games are people / animals and only monsters in the players eyes.
Or it was just something arbitrary put into the achievement list of the game, which has no impact whatsoever on our interpretation of the "mythology" (if there even exist one that isn't fan-constructed)
People put way too much stock into something I'm sure is not intended to be interpreted or understood as coherent or intentional. Especially with something as Silent Hill, which has had several different developers, all with different ideas about whatever fictional world they are creating.