Stupidity: Towards the end, one of the soldiers (Kieran) gets trapped in the store with a group of child hungries - he was injured in an ambush, but is still lucid/human. He threatens them with a grenade, even though he knows that throughout the story they are mindless and hardly capable of reason. Fair enough as a bad character mistake, but then to allow the hungries to close in on him and not detonate the grenade is inexcusable. Soldiers are trained to fight and, to a degree, accept even death, to obtain an objective or protect the others in their "unit." Dying anyway, he should have blown up the hungries to at least whittle down their numbers.
Stupidity: When the army base gets overrun the girl jumps on a soldier and starts biting his neck. The woman soldier that's with him aims her gun at them but doesn't shoot, knowing full well there's no hope for the man. There is utter carnage going on all around them and instead she tries to help him and gets attacked in the same way. This could be a plot hole as well as the girl in question is the one that holds the key to survival.