MAD
MAD in games is what ultimately kills multiplayer competitive games for me. You'll start to notice particular characters or weapons suddenly taking center stage, and being very hard to play against. Once you figure out their weaknesses, you notice it takes a significant amount of effort to deal with this compared to a 'regular' character/weapon, making you vulnerable anyways to other players. Thus, the OP'ness of something becomes apparent.
Players using these clearly overpowered selections, then talking trash isn't so much infuriating as it is just disheartening. Say something back, "Well I mean, you are playing anubis... he's sorta easy" and you'll get bombarded with "cry moar" et. al
So, you get pounded by the broken selections game after game. Talked trash because the opposite player thinks they are hot stuff, and nobody who understands game design sees the ridiculousness at hand. Developers just don't care because they are catering to something else and have other interests than a fair game.
Thus, MAD. Mutually assured destruction, if you can't beat em, join em. Now everyone picks the OP weapons, a meta is born, and there's no more creativity and fun in the game.
It's the only thing I really disliked about multiplayer games. In example, it's why Battlefield is full of snipers ALL THE TIME. "These people keep sniping me and ripping on me for getting insta-killed by them, so I'll just become one and insta-kill them back and see how they feel!" thus 50/64 players are recon.
Bad game design everywhere, and it just sucks. It's not the players taunting, it's the bad game design and balancing that leads them to be stupid and taunt.