If you can't handle a few words on an computer screen without actual face to face interaction you have serious confidence issues.
It's about someone not playing properly or making your team not play properly BECAUSE they are raging. If some moron is repeatedly typing "GOD you guys are so STUPID WTF are you doing" "KILL MORE THINGS" "WHY DONT YOU CHASE WTF" "/all lol my team is so stupid gggggggggggggggg", he's not playing the game properly.
If he is not playing properly and starts to type, rage, afk, or do idiot things to not play properly, now my *team* will also start to do these same stupid things, either replying back to him, getting caught in fights because he's *****ing so much, not being able to win fights because he's not there or runs ahead like a crazed albatross and dies trying to get a kill, or because he's not playing properly and putting down wards.
Face it. When someone starts to rage, they become a worse player. Raging doesn't make you better. It makes you worse. It takes away your concentration on the game, it hurts your teammates playing ability, it makes your team lose. Yet, some people still live in this fantasy world where hey, if you're angry about something, you should YELL AS LOUD AS YOU CAN AT YOUR TEAMMATES because this will magically improve everyone's playing ability, knowing that you're pissed as hell and not gonna take it any more.
News flash: It doesn't. You just lose more.
Where the hell have people ever seen a professional athlete become better at what he's doing because he's pissed off and swearing? I always wonder that. There is no sport, activity, or game in the world that you become better at by being angry, unless your profession is a berserker and you wield a giant claymore and cut off the heads of your enemies.