Too much is decided by having one really good or really bad player on the team

Honestly, it ruins the match. One person being able to single handedly carry or sink a team means that the amount of control most players have in most matches is very minimal and really just luck of the draw. I think this is a huge fundamental problem with the game.

Because of this, and players knowing this, it makes it even worse--players just give up when things look bad, creating a self fulfilling prophecy of loss.

The reason the community is toxic is because this game design itself is inherently toxic. You can punish me all you want. I am never going to not be upset that I played well and one awful teammate ruined an easy win. And I'm never going to not want to tell him that. Yup, I'm SO toxic for that. The nerve of me being upset that some troll just wasted my time. Please Riot, correct me.