Agreed. It doesn't matter if it is 1 time or 10 times. Intentionally feeding or going out of your way to ensure your team loses is wrong. If it is one time, give them a warning so they know they are being watched and that it is not okay. If it happens again 2 months later? Ban them for a week. Scale up consequences as needed from there until they learn to be a decent human being and teammate.
I get tired of people who get their feelings hurt and then decide to throw the game away or flame you or do everything in their power to make the match hell for you and call you toxic because you told them something other than "gj". Asking someone to ward when they push out or to group up isn't toxic or unreasonable. Explaining (nicely) how doing 1 thing differently could have resulted in X instead of Y happening is not toxic. Grow up and learn to take some criticism people. It's one thing if someone is just being salty and calling you trash/feeder/something worse; it's another thing if they are offering valid advice - which you should probably already know and be doing in the first place. The fact they explain it to you without flaming you shows they want you to succeed and are trying to be helpful - responding with cries of "toxic", flaming every little mistake they make, intentionally screwing them over at every opportunity, and throwing the game harder are not excusable responses.
I posted this tldr not because I want sympathy or anything. I posted this to show how blind people can be to their teammates contributions and how horribly selfish and childish the responses of an ENTIRE TEAM can be to someone for speaking honestly but critically. Hopefully it gets more people to think about the TEAM first and less about making themselves look good. I sure didn't play the game perfectly, but everything I did was to give my TEAM the best possible chance to make good plays and have advantages.
https://boards.na.leagueoflegends.com/en/c/player-behavior-moderation/TTWp7F7I-gold-logic-please-tell-me-how-im-the-mentally-retarded-renekton-in-this-game?comment=0000