Very fine rhetoric here with excellent questions to ponder.
Should we be preventing people from committing violence to themselves?
The short answer is no, because it is impossible.
Philosophically, as one who overcame addiction (I consider myself a veteran of the Crack War as I had a 8 year run ending in 1994), I say no resoundingly. I am a libertarian. I believe free people have the agency and right to harm themselves. Most of the time I harmed no one but myself. However, in the end this was no longer true and the law handled that like it should have. Now if I had a child at the time, I would have hurt him with my selfish behavior and that is another thing.
Fine lines here.