Fridge Logic: How exactly can Shidou get away with kicking a slightly injured guy in the face and leaving him for them, in full view of the bus everyone else is ready to escape on, and not be instantly branded as a monster by the 5+ or so people that would have had no excuse to not see him do it? You would imagine that with a group of survivors moving towards escape, someone would be paying attention to make sure they all got there safely.
Due to the confusion, it is not likely there is anyone paying particular attention on him. Even if someone glanced, it is not really possible to tell if that injured person is not one of "them". And it's not like they'd be able to check on everyone, since they were quite a distance away from Shidou, and they were preoccupied with things like killing any nearby zombies, or in Rei's case, arguing whether to let that monster get on the bus. But then again, almost everyone who isn't a main characters is a selfish, incompetent dumbass, so...
Plus, they had no way of knowing if the student was infected or not. If they think he's infected, suddenly Shidou's decision looks like him trying to Shoot the Dog to create a distraction so the bus can get away.
Fridge Horror: This troper was just in tearjerker section of anime and it implied that the gym teacher who killed himself had a baby. What happened to the baby? Shit, think about it, if children can become the undead, what's to say that infants can't?