If we knew what we did in situations like these - we wouldn't all be so active in these forums.
I wish I would do the following: check wheter I had a very good chance to save the clerk from the threat, pull and aim very slowly and silent at the back or the side of the BGs head and then shoot well aimed untill the BG stops moving at all.
I also wish that I would only escalate things and open fire if my chances to end up holding the only gun fired were excellent.
I wish that I wouldn't end up regretting why I didn't save a shot clerk athough I could have - likewise - why I started a shooting that killed innocent bystanders.
Tough question. Being a lawyer the answer just is - "that depends"
Si vis pacem - para bellum
If you want peace - prepare for war