In the movie "[*High Noon* (1952)](http://www.imdb.com/title/tt0044706/)", 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?