How Star Wars VII invalidates the idea that Storm Troopers can't hit what they aim at

Minor spoiler, obviously: At the beginning of the movie, Finn doesn't shoot during a massacre. My first thought was this was a mistake on the part of the writers—it would be more discreet if he shot over the heads of the targets. But then I realized that would mean Finn was being cunning rather than viscerally unable to slaughter defenseless people. And then I remembered that in actual combat,