Risk of doing business. Most small businesses fail. If getting looted is the cause, that's unfortunate. The risk is always there.
I'm not going to kill another person unless I'm sure they mean to kill me. That's just me.
Nobody is trying to make you agree with me.
Apart from the moral issue of killing someone needlessly, I think you might find that in most jurisdictions, it's a legal nightmare for the shooter. You would more than likely (depending on circumstances, evidence, witnesses, etc,) regret it.
I hope you never have to find out.
That's a big "if", and nobody mentioned a civil suit, but you raise another reason it might not be economically smart to gun someone down for stealing a few dollars worth of stuff.
I consider looting a business that I owned, the same as breaking and entering my home.
If I'm there and someone comes in through the door or a window, they're going to go back out the same way; just weighing a little bit more, from the lead.