This is just my experience over the past 5 years I've been playing this game.
It's the impression I get every single game. I don't know where this mentality comes from where every mistake is unforgivable and having a bad game is just the worst thing you could possibly do to your team. It's such an end of the world thing to people when someone is having a bad game and they try to make sure you know they are disappointed in you and pretty much hate you for it and your entire family with it. :D
If someone asks me "Should I play League of Legends?", I tell them "Only if you plan on playing with friends, otherwise no, I don't recommend it."
It is about the least fun experience as a new player in all the games that I've played. None of them are this toxic when you're playing with strangers.
P.S.: This may seem like a cry post, but I'm just speaking the sad truth.