Nevri:
Ooh, I was trying to imply that maaaybe the guy saw the message, knew of where our little GGs (Gakkou Gurashis, lol) were and tried to infect them because somebody higher up may have ordered him to do so. He may have had a change of heart mid-flight and just gave up, not wanting to return to where he came from without fulfilling his task. Again, I don't remember it being brought up later, so my guess is as good as yours!
However, most likely it is just a cheap way to get them to leave the school and develop the characters simultaneously. They could've left the school solely because of the graduation ceremony, but that wouldn't show us anything about our GGs. The result of the helicopter crash was that we got see Yuki as somebody who is much brighter than we originally perceived her to be. She is well-aware of their situation and will take off her cat beanie in dire circumstances (and she even fends off a zombie in this volume... at the expense of her sanity). Meanwhile, Yuuri is shown to be somebody who cannot handle pressure and is slowly starting to lose it. Kurumi's fatigue and zombie state becomes more visible, while Mii-kun is becoming a bit more like Yuki (more trusting, optimistic and positive).
I think the helicopter crash was relatively important in that it exposed Yuki and Yuuri's true characters. Plus, we saw in the earlier chapters that the teachers were even given unopen booklets in preparation of such an event, which implies that the zombie outbreak could've been planned by an organization (perhaps the government). What I was trying to get at earlier is maybe the helicopter guy was from this organization, and their plot is to spread the disease even further. But I actually don't know the answer, so we'll see! <3
