Acting out in murderous ways

First there was this.
Then yesterday, this.
My friend Bebe has a blog post about the former.
Whose responsibility is it, ultimately, to thwart such acting out?
Certainly, there have been recent incidences where airline passengers have subdued potentially dangerous behavior. You must act then. But what about more "benign" harms? You have a friend who's self-destructing but mainly hurting only herself. You can gently confront; but I'd argue it's the friend's responsibility to ask for help and receive help.
What say you?