The scientists of Equestria have made a dimension jumping device called the D.J.D. (that's its name because the author didn't have any creative names!), when they turned it on, the bronies and brony haters of one universe come into the Equestrian universe. While both types of humans showed compassion (or something near kindness like not being a killer) at first, but then the haters "accidentally" got hit by a spell that would force them to make violent decisions!
Only a handful of the thousands of humans are not corrupt, but that's not a whole lot! One human appears one day proclaiming that he would save this world! However he didn't give us his name, so he's going to be called Wanderer.

I found Rainbow Dash riding on top of Applejack here.