Religion.
It's done more harm than good.
Cases in point:
The crusades
The Dark Ages
Parents, in extreme cases, disowning children for not sharing their beleif
The only good thing it brings is a comfort to the truths of reality. I would have much rather people finding these things out, instead of lazing around and saying "oh life works like this".
Or just get rid of the people along the lines of the people trying to push SOPA and such