Number one, "Not worth his salt" was actually an ancient Greek term, pre-dating the Romans, referring to slaves that worked in the salt mines . . . the worst slaves were sent to the salt mines, and the worst of them were "not worth their weight in salt."
This passed on from there to a euphemism about all slaves to a euphemism about any person.
Get off your high horse, Heather. In case you didn't read the article (obviously not) it's GUNS FROM AROUND THE WORLD. THROUGHOUT HISTORY. And can't you see that from the US, whiny little pissants who want to moralize everything they can get their grubby little hands on look like complete idiots when they spew out their little comments on everything. (Not that I feel that is the case in the UK, and apologies to all the normal Brits over there who enjoy light reading about HISTORY like the rest of us!)
This passed on from there to a euphemism about all slaves to a euphemism about any person.