OK, in my past experience I've had not one but TWO situations that I considered to be major "double-you-tee-eff" ones, that revolved around a bad player or players. In both cases, it was players who started out great then turned bad.
This is the oldest incident. The punchline of it is I'm not quite sure we've heard the last of it. Call it a gut feeling.
Long story. Pull up a chair, get a drink. Hopefully, this story fits the criteria. I think it does.
Once upon a time, at GenCon 20, I was introduced to a guy (let's call him Chicken Boy, or CB for short), and his con-babe gamer-girlfriend Jade (not referencing PvP, her eyes are green). We all hit it off nicely, and it turned out that CB and Jade actually lived in my area. Naturally, I invited them into our game. Jade and I had an attraction, but I was loyally married at the time, and well, Jade and CB were exclusively dating. So, those feelings and ideas were pushed aside and we resolved to be friends. OK, so far so good.
Time passes, and it turns out that CB makes it to the game way more than Jade. No prob. He's a nice guy, and a very good gamer. Eventually, though, Jade and CB break up, and it was Jade's idea and initiative. The parting seemed amicable. Jade now makes it less than ever, but we stay in touch. CB is now one of our regulars.
My marriage ended and I ended up dating this girl, let's call her Brunhilda. We ended up moving in together. Brunhilda also gamed. As time went on, it became apparent that Brunhilda and I were a bad match. We broke up, and she began looking for a new apartment. During that interim period, she asks me if it's ok if CB calls her while she's still living with me. Although I think it's a tacky situation, I'm kind of relieved that I'll be spared the guilt of a breakup, besides, how can one say no in a situation like that, without looking like a Richard Head? The upshot of it is, CB and Brunhilda end up dating almost immediately.
I'm actually cool with this, after a fashion. I'm free of Brunhilda, and she and CB are happy, and they still game with me and my group. We're all still friends. Nifty! Unfortunately, Jade's going out with someone, so I just do some casual dating around.
As time goes on, CB starts telling stories that his relationship with Jade wasn't all it was cracked up to be (cue dramatic voice: Yes! Now it can be told...). He calls her abusive (she's 5'3", he's 5'10" and a black belt) and claims she threatened him with a sharp object one night after she lost her temper.
As time goes by, CB begins to change. He becomes crankier, cheaper, and complains more. He and Brunhilda orchestrate a threatened walkout because another of our gamers was getting annoying (which admittedly she was, but in retrospect, not fatally so). This woman tended to flaunt a lot of her material possessions, like several thousand-dollar rings, or very expensive computers. Faced with the prospect of losing four gamers, I confronted her (a friend of more than ten years), and basically asked her to step down. Stupid me. She did, and boy were her feelings hurt.
Then, Brundhilda and CB began bitching about this other guy in our group. A great role-player, he wasn't a very cheerful or happy person, and tended to make quiet snide remarks a lot. Brunhilda was also an extreme left-wing type, and this guy practically worshipped Rush Limbaugh. Again, they put pressure on me to show him the door, again I stupidly caved in and asked him to leave.
Then, Brunhilda and CB began complaining that my two oldest kids were in the game. Out of fairness, not everyone likes kids, and the kids weren't around when the campaign started. But I can say, wearing my DM's hat and not my parent hat, that the mistakes and verbal gaffes they made were nothing worse than whatever I've heard or seen from other gamers. Well, that's where I drew the line, and refused to relegate my kids to the bedroom halfway through the gaming sessions. So CB began setting up another campaign, where he would run it, and began inviting my other gamers to go to their place instead. Our campaign was still intact, mind you, but now cracks were showing...
After two years of silence, Jade and I resume contact. As it happens, she's broken up with her jerky BF, and I was also unattached. We go out to lunch one day and hit it off like we always suspected we would. The next day we go on a date. Instant connection here. After ten years, we can finally date! Hoorah for us, right?

CB and Brunhilda have each other, Jade and I have each other, so all's well that ends well, right? Riiiiiiiight.....
When Jade and I begin dating, I invite her into the game (show of hands? anyone see where this is heading?). CB has a crap-fit, and insists he won't game with her. Apparently, the "emotional scars" of that one time where she verbally tore him a new one and allegedly threatened him with a sharp object are "too fresh" and can't be forgiven. Mind you, we're talking about a decade ago at the time.
CB and Brunhilda leave the group. CB then sends out a mass e-mail to everyone, explaining that he was leaving the group, cited Jade specifically as the reason, but nobly announces that he won't say why, but invites people to ask him about it later. Wow. What a guy.
Jade tries to write him an apology, clear the air. He completely refuses to acknowledge her. Jade and I talk it over, and she admits that she lost her temper like that once, but no animals were harmed in the making of that incident

I also discover why she lost her temper, and boy was it legit. It also turns out that CB was just as toxic for her as she was for him. In other words, both were to blame for what was essentially an unhealthy relationship. But to hear CB talk about it, she's Lizzie-freakin-Borden, and he's some innocent victim out of a Lifetime movie. Oh, and incidentally, I've yet to hear or see any indication that he explained why she got so pissed off with him. Course not. That would imply he's not as guilt-free as he's painted himself.
So, CB and Brundhilda leave. When the dust settles from all of this fallout, my group is basically decimated. Around the same time, I move from the Boston area to my present location. A few of my gamers (one of them a very old friend, let's call him Kedar) decide they don't want to drive an extra hour, and they decide to game with CB. And though they also claim neutrality in this little pseudo-feud, they apparently have listened to his side, and don't want to hear Jade's side, because, well, it's all in the past and what's done is done. Huh??? Kedar also tells me that this is the sort of thing that happens because I decided to date my friend's ex. Huh again? Who dated who's ex first???
I decide that I really don't want to be in contact anymore with CB if he's going to diss my lady and in essence refuse to even acknowledge her existence. I don't want anything to do with him till he removes his head from his posterior. Brunhilda can't fathom why I'm doing this, since, according to her, "this is between him and her, not you." Apparently, the idea of backing your spouse up to this degree is an alien concept. Kedar has also taken me to task about not talking to CB. And never once did Jade ask, imply, guilt, threaten, or anything else, to get me to not talk to CB.
To this day, CB and I don't talk. Kedar and I don't talk. Brunhilda and I don't talk. Jade and I are very happy together and I've yet to see a melee weapon deployed!

We also have a healthy, friendly, robust gaming group of people who genuinely like each other.
The writer part of me thinks it's hysterical that CB and I, over a long period of time, eventually wound up with each others' S.O.'s. It sounds like some romantic comedy starring Sandra Bullock and Hugh Grant. Unfortunately, we can never get together and laugh about it, because he's clinging to this victimhood thing from an incident that at this point is over a dozen years old.
What an effing ass....
PS: Thanks for letting me vent.