Fridge Horror: Invoked. What happens when the people we trust the most—our parents, our children, people we depend on for safety—go completely off the rails?
In another sense, a comment near the end that what the Crossed do isn't anything that ordinary humans aren't capable of. That's scary as hell.
In fact, that could be considered a running theme throughout the Crossed saga. In all the story arcs one can see non-Crossed characters engage in Crossed-like behavior. Some examples are obvious, eg. Harold Lorre & Addy's father, while others are more subtle. This of course reinforces the concept that all the Crossed virus really does (apart from the facial rash) is strip its victims of all norms and concepts of civilization, society and morality, with some people already being like the Crossed even without the virus.
From Wish You Were Here: The revelation that the Crossed virus is not only capable of being transmitted through the flesh of carrion eaters (when's the last time you had some fish lately?), but is also apparently mutating into something even worse.
In Volume One, a flashback details how Cindy's group met a policeman at the start of the outbreak who had a Crossed locked in the back of his police car. The cop claimed the Crossed had said things about his wife that the Crossed had no way of knowing, and wanted to interrogate him further. Cindy claims that the Crossed hasn't said anything that he couldn't have deduced from a cold read of the cop and they discount the cop's theory. Flash-forward to "The Thin Red Line" arc, where the initial Crossed infectee in Britain does gain access to information that he couldn't possibly know, and it paints a more disturbing picture. Are first-generation Crossed actually, in some strange way, clairvoyant?
Because the average Garth Ennis protagonist, i.e. Cindy, would be in about as much danger during an actual zombie apocalypse as you are during a particularly rowdy soccer game. She'd hand out two thousand clean headshots and go put Patrick down for a nap.
Because those type of survivalists generally take into account only one kind of zombie, the slow-moving hungry-for-flesh type, and boast about how well they would do against them. Ennis just gives them a foe they have no chance against, and no amount of bug-out-bag preparation or choosing the right kind of machete will save you.
But the point is that the crossed aren't zombies, so the lesson(You couldn't survive in a zombie apocalypse) still doesn't apply- the survivalists aren't saying "I could survive psychotic attacks by beings capable of thought in huge numbers" they're saying "I could survive a ZOMBIE attack, by ZOMBIES,", which the crossed ARE NOT.
They are, though. Just a different type of zombie. In fact, being alive and merely berserk, they're probably much more realistic and likely zombies than what all the "shoot 'em in the head" nerds are prepared to face. The lesson isn't "you couldn't possibly survive in a zombie apocalypse", it's actually "you couldn't possibly survive in a zombie apocalypse if you automatically assume that the zombies you're facing are going to be or act in a certain way", which is most certainly true.
The fact that the zombies are more "realistic" makes the whole thing even more self-defeating. Part of the zombie stuff that's so engaging is thinking about these fantastical issues and how you would survive them. This isn't people talking about how they'd survive a hurricane, it's a particular brand of fantasy. Making it more realistic defeats the point of the exercise entirely. Saying nothing of how realistic a mystery condition that destroys all higher reasoning that still leaves you with the ability to use tools and drive vehicles is anyways.
And as an addendum to the above, those "armchair survivalists" who enjoy planning for a zombie apocalypse almost never talk about anything other than a Night of the Living Dead / Walking Dead style slow-zombie apocalypse. They don't talk about fast zombies, smart zombies, rage "zombies" or anything like that because it's not part of the fantasy. It's really more the equivalent of getting mad at an armchair sports coach type baseball fan who always says he could do it better than "X" coach could and then trying to prove that he couldn't by forcing him to coach a soccer team. Then smirking about it. But a more proper answer is the one directly below - strawman and author tracts that make no sense. It's on the level of the ending to the comicbook version of Wanted in making no sense.
Because, frankly, Garth Ennis is not above writing Author Tracts or using The War on Straw as part of that. He can't find a viable counter-argument for those who make arguments for "classic" Romero-style Zombie Apocalypses, so instead he moves the goalposts by replacing the iconic mindless shamblers with a literal Hate Plague that turns the infected into Axe Crazy killers without robbing them of their human intelligence.
It's probably more accurate to say that the "zombie apocalypse" angle comes up in that issue as more of an immediate indicator to the reader that this is not that kind of story, where the first reel begins with our heroes on the ropes and they're going to make their comeback soon. The one guy who actually mentions taking the world back from the Crossed is a moron who's steeped in science-fiction tropes and who dies for it. The Crossed are thus established as something other than power-fantasy cannon fodder, and it doesn't hurt that he got to thumb his nose at a particularly irritating brand of fan while he was at it.