Well now how about this info? From the force awakens we hear Finn say that they check each storm troopers rifles to make sure they fired, not where they actually hit their target... Another would be from rogue one. Chirrut imwe starts chanting his mantra and is protected by the force until his task is completed, so it's quite likely that the force is able to divert shots in order to protect those necessary. [spoiler]a theory I quite like is that the storm troopers on the Death Star and such know that Luke and leia are Darth Vaders children. Now what would happen to the trooper if they had killed one of them?[/spoiler] Also if storm troopers have such terrible aim then how is it they're able to easily take out so many rebels? The attack on hoth would be a good example that they're not quite as incompetent as it may seem...