Sorry, our laws don't permit me shooting a perp to save a stranger's life. In every state I'm familiar with, you may use deadly force to protect a third party from death or serious bodily injury. There is no clause about not protecting "strangers" vice people you know or family members.
In the state of Oklahoma the Lethal Force laws specify you're only allowed to protect family. The sole exception is if it's an employee. It's been hashed and rehashed locally and the concensus is to only protect you and yours.
There have been no test cases, however.