I firmly believe that, the reason for community being toxic is the game mechanics and not only players it self. Even if you had perfect team mates, the mechanics are quite frustrating for some people. Both in game and outside i.e reporting people and then seeing them still playing, LP grind and etc.
I mean League of Legends is a MOBA and thus it has certain mechanics expected of MOBA. Farming/Rotating/Picks/Bans and etc.
In a game where an afker/troll/griefer/ feeder/counter pick/etc, can ruin your experience is achieved so because game is designed in a certain way.
I'd like to hear your thoughts.
Now I might not have articulated my thoughts properly but I hope you got the general gist of what I'm saying.