For me, I think it started around when 9-11 hit, I was only a kid at the time, and knew very little about how the world worked...
The year after that, I learned about the Holocaust, and from there, it was all downhill.
Now, I KNOW I shouldn't be so cynical, and that "not all people are evil blah blah blah".
The reason I dislike humanity is because I know what they are capable of, not what they are like. But that's just me...