This video ought to be mandatory to watch every month or you won't be able to play.

In this day and age, a lot of teammates would rather fight each other and not the enemy. This has caused a lot of grief in the community. I'm pretty sure they had the same problem back when this video was originally published, but that was a long time ago and it isn't exactly on the front page of Riot's YouTube channel anymore. I think if everyone watched a refresher course on proper gaming etiquette they would improve their behaviour by even the slightest. I know Riot cares about eliminating toxic players but even they know they could do more. Even if it weren't made mandatory I would suggest that at the very least make this the front page article when you log in to the game once a month.