Don't Join in on the Hate
When someone starts insulting a player over the mistakes they've made, there are times in which the rest of the team will join in and bash the ally as though that single player is the cause of all the suffering in the world.
Don't, just don't.
Instead, it's better to turn around and tell the flamer to stop insulting your team mate. If someone who's made a number of mistakes knows that someone on the team is understanding and caring, it might be enough to lift their spirits and pull through with you. And when only one member on the team exists who insists on insulting others, they will be made to realize that theirs is the unwelcome opinion.
Earlier today, I saw one player say to another, 'You shouldn't play ranked. You shouldn't even play. What are you?' Fed up with his negativity, I replied and told the player, 'He's someone who might play better if you eased up off his back.' The offensive player spent the rest of the game without insulting our ally again.