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.
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.
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.