A hero is someone who protects the group's interest instead of his own even if it's at his own expense, risking to die for them. Some people have been raised to act that way, most atheists arent raised that way. When you come from a family where parents werent even able to stay together in the interest of both themselves and kids, obviously you re not going to learn how to do much for others.
Anyway, you dont know how you re going to react in such an event. In fact it all depends how you were raised. Im sure some happy kids loved by their parents actually might have attempted to help other kids or adults. I see kids coming from stable (rich or poor) families spontaneously helping other kids or adults once in a while.
These behaviors are normal, logical, especially coming from kids. One teacher locked herself and kids in a room. All of this is for real, it hasnt been invented to sugar coat anything. Fact is some people will react one way, while others will react another. The way someone reacts is conditionned by the way he or she was raised.