I think the assumption in the OP is "If you could shoot LEGALLY". Shooting a 9 y/o kid steeling a candy bar is far from legal. So why did u bring that up WA?
IMO if it is legally right to shoot a "BG" then it should morally be too. For me, the term BG denotes here some one who is in the act of a violent felony with third party. We have plenty of BGs here who "steals" (read rob on gun point) one off his/ her possessions specially cell phones, wallets etc & are pretty much trigger happy.
If I see them doing this with someone, will I shoot? NO but IMO since at that time it would be legal to shoot them, it would be morally right too. I may have my other reasons for not getting involved, but not because of "the moral duty to retreat".