I've seen that behavior in a lot of games as well. But I've played some fantastic matches with some really nice people. I mostly ignore what's going on in the chat while the game is playing, maybe joke around a little if something funny happens, but harmless fun and not trolling or anything like that.

Leavers/feeders/quitters/surrenders/afkers are so annoying. I've had my team do well and then someone initiates vote for surrender, or complain we're going to lose etc, then we get a few good teamfights and turn it around to victory. Or the other way around, where you have cocky people going for an "easy" kill, only to get ganked and then then the opposing team turns it around to their advantage.

Shows that no matter how bleak the outcome, or how well it seems to be going, don't give up. Keep farming, support your team and don't make mistakes!