welcome to the game my good sir, ever since WC3 it's wandered into a sort of exploration on morality during wartime. that's not the only thing WoW is about, but it's one of the guiding lights of the story. so yes, both sides so some pretty dark stuff and both sides are pretty evil, they just choose different flavors of evil to indulge in and then dip themselves in their respective flavors of goodness and staunchly proclaim that their good outweighs their evil.
personally i think the Horde has a slight edge in the good department as most of them (ok maybe the Forsaken don't agree and i don't think the Goblins moralize at all one way or the other) consider many of the things they've done and do to be evil but try to atone. the alliance overarching seems compelling to stick to a "the ends justify the means" mantra and carry on.
so yes, what you did is pretty atrocious. it's not the first, nor the last, nor even the worst thing your character or faction will do in WoW. the Horde isn't any better(expect maybe subjectively) nor are they any worse (again expect maybe subjectively). you will both continue doing evil things to each other and then try to parade how good you are (maybe you save an orphan dog kennel from burning down. i don't know. earlier that day you were burning down some weird building filled with wolf pups so they wouldn't grow up to help the Horde...all in a days' labor)
it's one of them circular thingies...what are they called?