I was researching SN this morning and found journal articles as far back as the 1930's describing accidental (or maybe just what was thought to be accidental at the time?) SN poisonings, most commonly in food kitchens and in families that were located in/lived in lower-income areas. Point is, we've known it to be toxic to humans for quite some time now. I'm guessing at some point, somebody realized that the unintentional (?) poisonings could be used as a method and it took off from there. I'm sure there's probably a non-N chemical that can be used in a similar manner, but I have no information on that.