I've talked to users and tried to lookup information on actual harm. Unlike, say, benzos, I didn't find anything showing, say, decreased intelligence or severe health issues. (There might be some immune suppressant effect, though.) Everything, as you noted, seems to look at users buying uncontrolled medicine and using unhygienic methods of administration. I dunno about studies of productivity. Anecdotally, plenty of people seem to find it helpful (they say out held them through the day).

>I killed someone for it

I can't see how this is anything but a pricing/accessibility issue. Otherwise there's just no incentive -- it's about as expensive a habit as alcohol if bought legally.