Answer:
I would be horrible. I don't know how else someone would feel if their friend had betrayed them. I would feel as if I wasn't good enough for them to keep a trustable bond. I was so worthless they didn't care if they betrayed me or not.
If the friend did it our of concern for my wellbeing then I'd be angry. Why couldn't they tell me before they betrayed me? It would not have caused as much pain. They should've communicated with me and explained to me their opinions and thoughts before going ahead and betraying me. I'd only understand if maybe I was being physically hurt.
Never. Unless someone is being hurt. Or if it includes a child. Other than that it's just cruel. They don't deserve the title of being a friend if they do anything behind their friends back. Even if it's a good intentioned back stab.