The Walking Dead (and its not unique in this regard) has as one of its basic tenets that it is not the zombies of its fictional apocalypse that represent the true danger to the survivors but other humans.
At the risk of sounding ridiculous, how realistic is this though? Groups coalesce when faced with shared hostilities. Zombies represent a genuine existential threat to humanity and possess no common social structures. They are the ultimate "other". No human can find common cause with a zombie. A zombie regards humans purely as food, we have more empathy for wolves or tigers.
While it is true that humans require the same resources of energy and food as one another the desire to co-operate to maximise the availability of those resources seems to be the historic norm. We don't routinely steal food from our neighbours and we never have.
In addition the zombies have no interest in the resources that humans desire. They have no need for food, shelter or comfort. Destroying zombies to reclaim resources seems more sensible than attacking other humans. Violence between groups of humans negatively affects all humans but violence directed towards zombies has no negative effects on human survivors and if effectively applied then humans benefit.
There seems to be no real reason why humans would not find it easy to find common cause with other survivors in a zombie apocalypse. Normal power relationships and dysfunctions would of course continue but the conventional boundaries of politics and nationality would dissolve.
But zombies are not really zombies. They are amongst the most symbolic of monsters. In the Walking Dead with its gothic backdrop of Georgia forests and southern heat you feel that the spectre that truly haunts the survivors are not the undead but instead the rotten issue of race.
It is not zombies that want to burst inside our barricades and undo all that we have made. Only other people have the capacity to do that.
Any zombie fiction that features the "humans are the real monsters" trope seem to me to be essentially reactionary. Seeing zombies as the masses who fail to share in a dream of, often, idealised American masculinity makes you realise that the zombies are less the Others than simply others.
The friction between the bands of survivors makes more sense as a portrayal of the fratricide of fringe movements be they left, right or those of feminism or animal rights.
You can trust no-one but yourself is their badge. Individuality is their shibboleth.
The survivors are not the heroic holdouts of a better time but instead the bitterenders spitting their defiance in the face of history and indifference with a hail of bullets and a stockade.