This player should have been banned, but he was pardoned. The Tribunal is broken!
The Tribunal was inspired by 'crowdsourcing' research. When Lyte was still a student, he worked with labs that developed games such as "Foldit" that took advantage of crowdsourcing to find solutions to extremely tricky tasks--when you read this research you truly appreciate how accurate crowdsourcing systems can be. In fact, the Tribunal is far more accurate than traditional Player Support.
However, the Tribunal isn't perfect. The design is based on statistical probability and random sampling and every system like this has outliers. This means that sometimes, a player has an egregious Tribunal case but is still pardoned because he happened to get a lenient set of voters; however, if the player is truly toxic, he actually is re-sent back to the Tribunal in a handful of games and is usually accurately punished in his second visit to the Tribunal.
Because the Tribunal is designed to identify frequency and consistency of toxic behavior, this is why it doesn't take into account the severity of the offense. If a player is extremely toxic he will quickly get banned multiple times by the Tribunal until he is permanent-banned; in fact, extremely toxic players end up permanent banned significantly faster than players who are 'less severe.' In the future, we may want to brainstorm ways to improve on the Tribunal so that it does take into account severity and adjusts punishments accordingly... but we're not there yet.
That player in my game was far worse than me. Why am I banned???
I'll keep this one simple: those other players in your game that you thought were worse are probably also banned. Nearly 100% of the time, those players were banned even before you were. If someone is being toxic, just report them and move on.
We strongly believe that it is not OK to retaliate. If a player refuses to cooperate and says something negative, then players should ignore the player and report them after the game. Players can ignore or mute players by pressing the “Mute” button on the Scoreboard [when you hit TAB] in the game.
If a player retaliates and says something bad back, then the two of you have made a negative experience for potentially the other 8 players in the game. Rarely do teams start working together better when players begin fighting back—great games only happen when players cooperate and don’t make the issues even worse by fighting back and forth.
http://forums.na.leagueoflegends.com/board/showthread.php?t=2411284