When we were teenagers, my brother started hanging around people who were getting him into serious trouble. He told me things because he trusted me and made me promise not to tell anyone.
I broke that promise and anonymously gave our parents enough information to intervene. He spent years wondering which friend had betrayed him. His life eventually went in a much better direction, but I still don't know whether I did the right thing in the right way. He trusts me completely today. That is the part that bothers me.