Before a match begins, a good Overwatch team will negotiate what roles people take during the round. While that's expected, plenty of fans still manage to be huge dicks about telling other players who they 'should' or should not play.
Jokester videogamedunkey has recorded a bunch of instances where jerks explicitly told him not to play a character, only to have Dunkey pick that exact character just to make a point. And guess what? He kicked arse anyway, as you can see in this hilarious montage. (Just so you know, lots of explicit language in this video.)
Sometimes, that Hanzo player will actually be the worst one ever. But hey, you never know — so be nice!