Actually even for a life and death situation this seems pretty uncharacteristic of high school kids. And it's pretty hard to believe the only alternative explanation for this phenomenon: namely that most are psycho-/sociopaths. Most characters are villainized in this manga for a purpose that I can only assume is shock value. As they say "writing is hard", seems this is doubly true for manga/LN authors. Not to mention that humans have very small bones both in the wrist and the feet.