Someone who plays NOTHING but Genji, and is actively participating in the game? Trying to move the payload, or whatever it is the map requires? That's competing against other players and trying to win. Someone who plays nothing but Tracer, or Hanzo, or Widowmaker? Same thing. Someone who only plays tanks, or only plays offense? Same thing. As long as they're working towards the map objectives, they are participating and attempting to win, so they are doing PRECISELY what the game requires of them. Don't like it? Maybe YOU should play Mario, that way you never have to cry about losing, and you can be lord of the game all you want without having to worry about 5 other players not realizing you're General fucking Patton and disobeying your orders.
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If the guy is in game, participating in the match, and attempting to do whatever the map's objective is, he IS playing as "part of a team". Playing as "part of a team" does NOT mean "doing whatever some dick called Draex wants me to do".