The Stormtrooper Effect attempts to explain why Stormtroopers are such lousy shots. tl;dr: They aren’t lousy shots. It’d be stupid to allow a minor character to kill a major character before the story moves along.
“Psst… See that guy over there? He doesn’t belong in this movie.” “But he played Han Solo.” “I don’t care. Unless he’s got a frickin’ laser whip, he needs to get off of my dropship.”