Why did Marshall shoot one of the villains before he committed a crime?

Question

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?

Answer 1

They have made it clear that when Frank Miller came to town that they (the collective group of outlaws) were going to kill the Marshal. They undertake that threat when he does arrive and that allows the Marshal to finally act. The clip attached demonstrates this.

There are also two other reasons:

1. High Noon was created in the era of the Hays Code - This was a film standard which movie studios voluntarily adhered until the mid-1960s. Under this code, movies were held to a ridiculously high standard where indiscriminate violence couldn't be shown and that any hint of law enforcement corruption must end with film punishing the transgressor, If Will (the Marshal) had shot the Miller gang or arrested them without charge under the "logic" of the Hays Code, he would have had to be "punished" at the end of the film.
2. Gary Cooper was an actor who had a great of "difficulty" playing roles which required a great deal of subtlety - There is a film from 1959 called "They Came to Cordura" where Cooper's character was supposed to have been in a prior homosexual affair with a character played by Tab Hunter. Although Cooper was growing ill with the prostate cancer which eventually caused his death, he also did not feel comfortable being perceived as less "manly" than he was normally portrayed and he insisted that his character be "toughened up" which actually undercuts much of that film's narrative.

Had Cooper's been written as ambushing Miller's men before he arrived or arresting them illegally, he would have likely objected strenuously. As it was, he had serious reservations about the perceived "socialist elements" of his High Noon part which is probably his best role.

So as soon as Miller and his men come into town and attempt to kill the Marshal, he is then "free" to commit violent acts against them.

Reference:

http://books.google.com/books?id=4OBepfVJOeEC&pg=PA301&lpg=PA301&dq=%22The%20Don%27ts%20and%20Be%20Carefuls%22#v=onepage&q=%22The%20Don%27ts%20and%20Be%20Carefuls%22&f=false

Answer 2

the Marshal shoots one of them dead, before they have committed a crime!

Not quite. One of them had just smashed a store window and stolen a hat.

Why?

A careful look at the video reveals that the bad-guy drew his gun first:

So legally, the Marshal shot in self defense.

And incidentally, the dead bad-guy also happened to be the hat-thief.

(For those that don't know: when watching the video on YouTube (instead of inline here), one can pause it and then single-frame advance by using the "," and "." keys.)