Yeah, that's fun. Ok, let's explore this excruciating subject with more tact that I usually ever have.
I was going to go on a long ramble about how you should take a long look at the situation and see where the blame really lies and yadda yadda yadda blah blah blah. Look, if things are so bad that you need this kind of advice, then you already know what's going on and you've reached the point of passive-aggressive avoidance. That hasn't worked either so you need to have a serious talk or the game is gonna die because it just isn't fun anymore.
Ok, I'm giving up on tact. I've written and rewritten this five times and I'm no good at phrasing this kind of thing lightly.
Look, just don't be a coward. If it's your job to have the conversation then have the conversation. Tell the player what the problem is and ask them to fix it. You can do that effective communicator crap where you avoid saying "You do this" and say things like "I and the group feel this way" and as long as you have a player who's so easily manipulated by bullshit pop-psych then, well, I hope that goes awesome for you. If the player has a mental level above twelve, though, then I suggest you just lay it out straight and ask them to fix it.
"Look, Kevin, if you keep killing every npc I set out for Amy's romantic subplot just to piss her and I off, I'm gonna set your character sheet on fire and throw you off the balcony."
Ok, maybe you should avoid threats. I'll try again.
"Kevin, that thing you do where you kill all of Amy's character's boyfriends? It hurts her feelings and it pisses me off. I don't know why you're doing it, it's not funny anymore, and it never really was funny, so, please, stop."
Hey, you could even have that same conversation in the touchy feely way. Watch.
"Kevin. Amy and I feel that when you killed Lord Alfar, Thomas the Courtier, Sir Faris, Duncan the page, and Angus the man who pulls the dung cart, that you were acting out. Our feelings are hurt about it, and I don't know if you did it on purpose, but please think about that before you attempt to brutally hack another of her character's love interests into small pieces, ok? hey, big guy, let's hug it out. No, don't try to hug Amy, she's armed."
Most times your problem players don't know that they're problem players. They wonder what the fuss is about. Most people who want to sit down and roleplay together are friends and friends usually don't want to actually truly piss each other off. Your average beef in a game is usually a wind up gone too far, or a bad habit that needs to be checked. If you do find out that the player is actively trying to hurt feelings and piss people off, then what the fuck are they doing at your table?
Here's the very small list of things I actually will kick someone out of a game for.
Repeated no-shows without warning.
Breaking lines and veils because you don't care about the other player's issues.
Things I would kick you out of my house and/or life for anyway.
"I don't see what the problem is. It's just a game."
Let me go off on a small rant about that last one. It's not "just a game." it's what we all took time out of our busy fucking workday lives for and made a fucking commitment to. We have relationships and work, and housework, and some of us children, and other hobbies and all sorts of shit to do because we're in our thirties and we've long since discovered that being an adult fucking sucks. So the fact that we cantake a few hours out of our depressingly busy lives to all sit down and do something for funmeans that it should be fun and that it should be free of fuckheaded problem children who make our time at the table miserable and make us wish we'd just stayed home and done laundry.
If I bring you a problem with your behavior and you blow it off by telling me it's just a game you're insulting me and everyone else who took the time to show up here. How much shit could you pull on a golf coourse before you got thrown off and would saying "it's just a game" help? Would that shit keep you in a bowling league? What about in a relationship? If you acted like a dick, diidn't take it seriously and broke dates all the time without warning, how would that go?
"Just a game." Don't waste my time. Get the fuck out and wipe the smudge your ass leaves on the door off with your shirt. If you're not willing to take it as seriously as the rest of the group, then don't commit to play in the first place.