When I was in high school my core group of friends was basically The Breakfast Club. One girl was, at the time, undiagnosed as bipolar and exhibited the type of behavior your might commonly associate with such a diagnosis. She was also the first in our cohort to come out as gay. I think those two factors made her a target, even if unfairly, for bullying. Our other friends, especially the stereotypically preppy popular girl, went after her mercilessly, and I spent a lot of time defending her. At some point, however, they succeeded in spreading lies about both of us to each other - creating a rift. I didn't start to outright mock her, but I did stop defending her. One day at lunch she was sitting at a different table and someone noticed her crying. I remember saying loudly, "I don't care!" so she could hear me.
Not long after that she tried to kill herself, but fortunately, did not succeed. I didn't see her again until prom, and basically lost touch with her for a year after high school. Eventually we talked and discovered that all the drama between us was 100% made up by the other girls, and we ultimately forgave each other. She just friend requested me on Facebook (I only joined a couple weeks ago) the other day. I haven't talked to her, yet, though.
At any rate, I never quite forgave myself for how everything went down. I think it's why I stopped talking to her after high school even though I really did consider her my best friend. I just couldn't face her, or what I had done. Even if I wasn't the "most responsible" I could have tried harder to stop it. I was never worried about fitting in or having others make fun of me. I was just mad at her for perceived slights, and at that age, I didn't really know how to talk to people directly about problems.
Ultimately, when I found out what our other friends had done, I basically lost all interest in friendships. I've been in a happy romantic relationship for years, but I have not made a single offline friend since high school. I don't really trust people anymore - especially because, even as an adult, I see those same traits in people that I saw in those immature high school girls.