In the movie "High Noon (1952)", when the Marshal is looking for some assistants to help him fight Frank Miller and his friends, one of the civilians asks:
Those three killers walking the streets bold as brass. Why didn't you put them in jail where they ought to be? Then we'd only have Miller to worry about.
and the Marshal answers:
"I haven't anything to arrest them for. They haven't done anything. There's no law against them sitting on a bench at the depot."
But when the four villains arrived the town, the Marshal shoots one of them dead, before they have committed a crime! Why?