@Killian_, you're making the very mistake that the comic is criticizing. The whole point of this entire goddamn story is that these kinds of rational plans are dumb because the people who make them tend to be Internet assholes who forget that living in the postapocalypse is a lot more difficult than pacing aggro in WoW raids. I mean, jesus christ, they have a single shotgun with two shells that's wielded by someone very inexperienced with the weapon. And stairs? What stairs? the entire inside of the malls are full of zombies. You could group up near the ladder, but then you consider that climbing a ladder is just asking to get your leg chewed on. Also, they have a limited supply of ammo. This is Japan, it's not like gun stores are particularly common. If they can get in, get some food, and get out, that would be ideal.
Also, you missed the crucial thing that the story has been throwing in your face: these zombies are human. They're not just Romero walkers or animated bodies. They have actual thoughts, are still semi-cognizant. Hiromi is the most extreme example of this, where we get to see her entire mind's eye view, but other zombies, from the ones who play shogi to Tekko who plucked out all her teeth, are still, on some level, sentient human beings. Killing is of course going to be necessary, but unnecessary, industrialized killing just comes off as genocide.