League in-game chat is a wonderfull thing. Rarely is it used for actual team strategy. Instead, its used for talking random stuff or in most cases, flaming. When people are angry at you, they will type away. Calling you names or blaming you for stuff. The best thing you can do is ignore it and not reply. It may get you mad reading whatever they say but you have to ignore it. It only adds to the fire if you respond. Practice this and you'll become less toxic.
It's easy to blame people for not being good. You have to realize that not everyone is going to be as good as you or better. There will be people who just aren't as good, or whoever they play agaisnt just happens to be better, it can't be helped. Even you, will play against someone better, and people will blame you for not being good. Its just a cycle, and after all its just a game.
Being toxic isn't just with words, it's also with actions. Giving up is a toxic behavior. Its not fun for other people that still want to try to win. besides you never know, you could still come back into the game. Anything can happen. Its best to fight till the very end. Although if even you know there's a chance you won't win, just surrender. Its better than giving up just because they got a double kill or somthing silly.