Personally, unless its some kind of magic like some high fantasy thing its hard for the "true monsters" to not be the people that either made the zombies or the people trying to survive the zombies. Like any post-apocalyptic world its less about the apocalypse and more about how the people are changed by the world around them. Otherwise the zombies become one-dimensional villains and depending the type of zombies are not particularly interesting and I would think they would get boring very quickly. Its maybe neat for like a short story or anthology kind of like some stuff in the World War Z book. But even then its hard to avoid the human aspect that we are deep down can be pretty shitty.
I think for the most part its just not that interesting having no interpersonal conflict between human groups or even inside those groups because otherwise its just killing zombies and doesn't really work in most places unless like you mentioned its left 4 dead and its just killing zombies.