The standard of using "deadly force" is quite simple and universal in US: A reasonable person believes someone is in immediate danger of death of great bodily harm.

A suspect who is escaping is usually not a danger to other people so it is illegal to shoot him.

However, if the cop believes the escaping suspect is going to kill someone and "reasonable people" agree, that would be different.

In the case of the movie "Fargo", the lady cop could argue that she knew the suspect is a serial killer and a danger to the public. It is not a clear cut case.