Hey, I had this same exact problem a couple of years ago! An old friend that had been in my game for over a decade was throwing tantrums when he'd feel challenged, or he'd just assume that any challenge that he couldn't instantly see a solution to was insurmountable. He would sulk, talk about killing other pcs' characters when they did something he didn't agree with (and he was supposed to be CG), etc.
After he stopped playing with us, every single player in our group came up to me at one point or another to tell me how much better the game was. Even dude's best friend.
Talking hasn't worked, time to try the boot. Talk to the dm and the other players; if they agree that he's spoiling the fun, or at least badly impacting it, the dm should have a sit down with him and say, "Look, Fred, we've talked about this before and it hasn't worked. Either you knock it off and keep it knocked off, or you're out. That doesn't mean that we don't all want to be your friend, it just means that you don't fit in our game." And the next lapse, boot him mercilessly at the end of the session. "Sorry, we've talked about this a bunch; you're making the game less fun for everyone else, so BOOT."