person reasonably believes the use of deadly force is necessary for self-defense against
(2) serious physical injury
(3) kidnapping, except for what is described as custodial interference in the first degree in AS 11.41.320
(4) sexual assault in the first degree;
(5) sexual assault in the second degree;
(6) sexual abuse of a minor in the first degree; or
(7) robbery in any degree.
(b) A person may not use deadly force under this section if the person knows that, with complete personal safety and with complete safety as to others being defended, the person can avoid the necessity of using deadly force by leaving the area of the encounter, except there is no duty to leave the area if the person is
(1) on premises
(A) that the person owns or leases;
(B) where the person resides, temporarily or permanently; or
(C) as a guest or express or implied agent of the owner, lessor, or resident;
(2) a peace officer acting within the scope and authority of the officer's employment or a person assisting a peace officer under AS 11.81.380;
(3) in a building where the person works in the ordinary course of the person's employment; or
(4) protecting a child or a member of the person's household.
If you are in the back of the store and the guy with the gun is at the front, your options of retreat just reached nil.
In this case you "should" be in the clear. You have reason to believe the BG is going to murder or seriously injure the employee.
The key thing to remember is "would a jury believe I acted reasonably"