Being "Nice" to your Teammates DOES NOT win you more games.
There's this huge delusion that Riot has created where "being a good sport to your teammates wins you games".
This is absolutely NOT true.
Being nice to someone on your team doesn't teach them the game, it doesn't make them gain experience. Now I'm not saying that players should just be a dick to every person but there's a line that has to be put down somewhere. When you have a player on your team that keeps charging into the enemy team feeding, teleporting to push into 5 enemies, ruining everyone's games... Someone HAS TO SAY SOMETHING TO THEM. It doesn't matter if you are just tilting them, they're already doing poorly. What good does it do if they can see the gank coming because they aren't distracted when they're going to die to it either way? If you don't say something they're going to keep doing the same thing forever and probably never learn.
There is no nice way to tell someone that they are ruining your game, I'm sorry if you think otherwise.
At high ELO's, you can see clearly that people already understand the game. They are intelligent enough to figure out what they are doing wrong. It makes sense that a player at a very high caliber wouldn't need someone else's input to know that they're messing up and that they need to fix it.
The same is simply not the same lower down the ladder.
These people don't understand what they are doing wrong, they don't even know they're doing something wrong to begin with. They'll keep overextending and dying to the enemy Jungler again and again. They'll have the enemy laner under their wing, but let them roam around killing their entire team without pinging they're missing or following to keep them in check. They'll stand around and farm or watch as their teammates die in a fight when they have the proper tools in their kit to win that fight for their team.
Ignorance is Bliss, you'll always think you're right if you never understand the correct way to build or play in any given situation. Even if you "nicely" point something out, these people will get offended. "I'm not wrong, I could never be wrong... my whole team is just feeders.. you guys are bad".
Not to overlook the fact that all of us make mistakes.
In the meta right now, solo carrying is probably more difficult than it's ever been. Without teammates who do their job at least half-assed, it's extremely hard to win. If you think it's just one single player, get over your brain washed self and gtfo. For years I've seen the whole "hey you're the only thing on your team that doesn't change". You're 100% right, in a scientific world that means any theory for winning would be impossible to figure out because you lack enough constants and have far too many variables. Every game your team changes and so does the enemy team, you have to play a different way every game with different build paths and items based on the situation you are in. If you are losing, it doesn't mean that YOU are always the problem. Whoever came up with that is full of shit. Sure you can always improve yourself to where you will do better but you can't control every factor of a game just because you play slightly better.
If someone on your team is ruining your game, you should never be punished for saying something to them. Granted, I do believe there is a point where maybe you could be going too far (like telling someone to kys, I do believe that's overkill and morally wrong to say to someone), but telling someone that you think what they did was completely stupid and that they should stop doing it should be acceptable (my opinion obviously).
In the real world, people don't hold back to tell you that you fucked up.. sure it probably pisses you off hearing someone tell you that you were wrong but it definitely never killed anyone.
If someone tells you that they think you aren't playing right try to play how they are telling you. Maybe they were wrong, but you'll never learn for sure if you don't try something new right?
Don't get butt-hurt because someone's upset at you, they want to win just as much as you do. Before you cry about how they're "toxic" in chat just calm yourself and think about if there's actually some validity in whatever they said. No one gets pissed off for no reason.
