With how so many stories like this end, I can't help but wonder why it feels like the author never considered how to end it. With a fight series, you pop up a big bad, deal with it, call it a wrap. Romance, you pick somebody and move on. They can more or less reasonably end at a moment's notice and allow for random/organic development.
With a story like this, there's a fixed duration (unless, say, there's a round two, but even then, clearly defined arcs). The ending's looming over you from the start, so why does it feel like they never accounted for that, and have to resort to random disjointedness? Even if they don't know what to do in the middle, they know where the end is: Sometime when there's only a few survivors left. If they need to end it early, they can kill off people faster but leave the planned ending intact.