My best friend and I “broke up” in January-ish. I wasn’t trying to force a break up, I figured it would be more mature and just plain normal to let things fade out. She had become involved in a romantic relationship that I simply could not stomach (partially because it was so very immature and toxic, partially because it was a reminder of a relationship I had been in myself that ended in physical altercation and divorce papers). I tried to be supportive, I tried to help when they broke up then got back together then broke up then got back together. I realized I was losing all respect for her. Finally she got back together with him again and I just tried to fade into the woodwork and I was sad but realized I did not respect her anymore and therefore, we just weren’t friends really anymore.
Then she pushed the issue - why aren’t you talking to me? I know you don’t think this is right but you of all people should understand because of what you went through… That kind of stuff.
I sent her the nastiest, most vitriolic email I’ve ever sent in my life. It not only ended the relationship, it napalmed it, then dropped the A bomb on it for good measure. It was built up months of my impressions of what kind of mother she had become. It was ugly. She responded with similar ugliness and threatened me.
We haven’t spoken since, and some of our friends chose sides even though they were not asked to (by either one of us). She has basically abandoned the group.
It hurt very bad for a short time. It hurt until I realized that anyone I could have such strong negative feelings for was not a best friend after all.
ETA - rereading I see it looks like I dumped a friend for being in an abusive relationship. It’s not that, it’s much more complicated. I did not leave a woman being beaten because she bugged me… just for the record.