I'm the backstabber, I guess?
I grew up with two friends from the elementary school all the way through college. We were a tripod through and through, going through everything from first girlfriends, smoking pots and drinking for the first time, and kicking the shit out of each other. The three of us shared nearly every formative experience together. We all split up for college, and while two of us jumped right into the fray, one started to become quite depressed.
He was angry depressed too, not just the "I hate myself" depression but the "I hate myself and every single person on Earth as well" depression. Combined with a terrible home life, he can't shake his funk and for years we witness (and hear about) terribly self-destructive behavior and a couple suicidal nights.
Over the years we all start to grow a little intolerant of our friends constant shit-show and need for coddling. Most of all, his whole "I hate everyone, I'm so angry and everything/the word" shtick. But he is our best friend, so we endure it. That is until one road trip in which I was charged with DJing. My angry friend always got every album of every artist we liked so naturally his phone was selected for DJ purposes. This was back when the Iphone 2 or 3 came out, a phone that I didn't own but was very curious of and excited by at the time (I easily excite). I was swiping around the main screens when the "Search" screen came up. I asked my angry friend if the function searches the whole phone, to which he replied it did. Cool! Now I don't even have to go to the music app and search through all that junk, I can pull up "One More Time" by Daft Punk by just typing it in! I typed in "one" and all of a sudden his emails start popping up with subject lines "m4m one night stand" and "m4m one time thing". Naturally I clicked show more. There were hundreds of emails all for craigslist gay hook-ups. In shock that my best friend of the majority of my life might secretly be gay, I blurted out "dah fuck is this angry friend?"
That ended not only the road trip but our friendship as well. We were literally kicked out of the car on the side of a highway and had to walk 7 miles to a rest stop and called a very expensive taxi. He rightfully blew up for snooping through his phone (will always regret clicking "show more"). We tried to talk to him about it all, mostly to tell him that it was OK and we still loved and cared for our friend regardless of who or what he likes to do. He wouldn't have it. Spit a bunch of crazy hurtful bile at us both in a couple of texts and that was that. Cut off all communication with us, and neither have spoken to him in over two years as he refuses to acknowledge either of us.
tl;dr accidentally discovered unstable best friend of 15 years was more than likely gay during a road trip, "outted/betrayed" him in the car, hasn't spoken to me in over two years.