Tired of Raiders

How am i supposed to sit back and relax in this game when the moment i turn my back to a Raider who we exhanged a "Hello" with just starts attacking me after i turn my back to them, or if i'm fighting a Bastion and then get blasted by these players?

And if i try to fight back then i'll just get put into lobbies with even more of these kind of players, how is that fair?