Very good thread with lots of thought-provoking input. Indeed, it can be very difficult and painful to stand by and watch others suffer from (any) illness with little or no hope for help or recovery.
Regarding the stigma associated with mental illness and substance abuse: For me, over time, the stigma has faded through learning, taking positive steps, and then really accepting that mental illness and substance abuse exist in my family. Truly, these illnesses are widespread (at least in and around the city where I live) and ostracizing and criticizing those affected is not constructive and won't make the illnesses go away.
For those that judge, I think back to my childhood and the words of an adult mentor, "When you point your finger at someone, there are four pointing back at you."