No one can diagnose another as an alcoholic.
Only the person themselves can, or perhaps a doctor specializing in the field.
"the difference between killing and letting die."
I think there is no difference. Guess its like the last Seinfield episode.
Or look at it like telling a lie, by ommitance. Its still a lie, you just feel better about yourself.
Look at it differently as in a child or baby.
You have a newborn baby, and just leave it alone, not feeding it.
Thats murder, not only in my eyes, but the laws eyes too.
Adults, laws become shadier.
Its best to warn others, and if they choose to disobey, so be it.
Such is life.
How hard are you willing to warn them, hold them back. There inlies a quandry.