Gun Control & RKBA

>Are there levels of threat?
Yes. To justify use of deadly force most places require that there be a threat of death or Great Bodily Harm [or Severe Bodily Injury, or some other similar wording].

>What standard do we use?
Innocence.
Imminence
Proportionality
Avoidance [or Retreat] in some jurisdictions
Reasonableness

>Does the shop keeper have a responsibility to not use excessive force.
Depends on jurisdiction

>Does the shop keeper have a duty to retreat?
Depends on jurisdiction

For a very good coverage I suggest The Law of Self Defense by Attorney Andrew F. Branca.
He goes into the details of the elements and standard pertaining to the use of deadly force and the elements necessary to present in a court of law.
A comprehensive index covers each state and the differences needed for a successful defense.

-- I am not a lawyer