Once upon a time, when the World of Warcraft was all shiny and new, things were simple: the Horde hated the Alliance and they, in turn, despised the Horde. It was a simpler time, but this wasn't just some game mechanic to facilitate PvP and hours of ganking, oh no, it was integral to the lore of Azeroth.
Now, it's also worth remembering that there's no black and white in Azeroth either. The Horde aren't evil for just being the Horde, neither are the Alliance all pure as newly fallen snow. Each faction has done its fair share of pure and evil acts depending on the actions of individual heroes and the motivations of their leaders.