The hardest thing to learn in life is that ultimately you can't change people. The only person you can change is yourself.
Others will make mistakes. They aren't TRYING to ruin your game (well, usually not).
Sometimes it just takes a shift in perspective. Try focusing on what you can do right, instead of what they are doing wrong. If they ask for help assist. If they ask for advice, then advise. Else, just play the game.
Also: Go into the game hoping to win, but not 'expecting' to win. That just sets you up for disappointment if you lose.
Sportswriter Grantland Rice once wrote, “Failure isn't bad if it doesn't attack the heart. Success is all right if it doesn't go to the head.”
Further paraphrasing Mr. Rice¹, "It's not whether you win or lose, it's how you play the game". Yeah, it sounds corny but it really applies - even to League of Legends. I'd rather lose with good and friendly teammates than win with a bunch of jerks. Same applies to professional sports and e-sports. Remember Tom Brady and "Deflategate"?
https://en.wikipedia.org/wiki/Deflategate
People ruining the game based on their attitude and breaking the rules can get penalized, suspended and/or banned, fined, etc. The rules apply to things like cheating (deflating a football, hacking, illegal enhancement on your vehicle in NASCAR) as well as 'behavioral' rules / rules of sportsmanship (personal fouls, badmouthing in League of Legends, mean comments or physical assaults in NASCAR, talking back to a referee, etc).
Karma says she always catches up to you in the end, right? Try to be the kind of team-mate that YOU hope you'll get in your games. The more positive your attitude in-game and outside it, the more it will become a habit and the easier it will be, and hopefully that positivity will spread to others.
That's all I'm trying to do as well. Spread the positivity. :D
See you on the rift, maybe.
¹ “For when the One Great Scorer comes
To mark against your name,
He writes - not that you won or lost -
But HOW you played the Game."
"Alumnus Football”
― Grantland Rice