Why is the community so bad?

After playing 5 ranked games tonight, 4 of them were ruined by people trolling. Not just having a bad game - running down mid to purposely feed my lane then afking.

IMO Riot needs to introduce perma-bans for people who feed on purpose to clean up the community. The game is a lot of fun but there are so many players who simply live to ruin your experience.

Thoughts?