Don't people want to win games? I guess they don't because if they spend so much time trying to put down their own teammates then they clearly don't want to win.
There is no benefit for flamming your own team that will make them suddenly play better, in fact flamming only makes them play worse.
Do people make mistakes in league of legends? Definitely, but then again so do you and even professional players.
Are some people better/worse at league of legends than their peers? Of course, but you are in the same elo as them so like it or not you are stuck with them for the game.
If you hate relying on other people's ability to play the game, then please stop playing league of legends and play a single player game where you are in control of most factors and can win everything.
I know this is a baffling concept, but this is a **team** game. Yep that's right, you are supposed to work together to win the game, not yell at each other in a psychotic rage.