Bumblebee
Commodore
Killbilly, to be perfectly honest with you I do not know enough about the subject to answer it. Sorry.
It can only be myself or a fellow on duty LEO. Someone can shoot someone sitting right next to me and I would not be allowed to respond with deadly force until I feel it's my arse.
"Use of deadly force" is often granted to police when the person or persons in question are believed to be an immediate danger to people around them. For example, an armed man in a shopping mall shooting at random without regard to the safety of the people around him, and refusing or being unwilling to negotiate, would likely warrant usage of deadly force, as a means to prevent further danger to the community. In the US this is governed by Tennessee v. Garner, which said that "deadly force...may not be used unless necessary to prevent the escape and the officer has probable cause to believe that the suspect poses a significant threat of death or serious physical injury to the officer or others."