I've played the RTS games and from what I can tell the Orc's originally invaded Azeroth in the first war, and then were the ones that started the second war by invading Lordaeron. By that logic, wouldn't the Alliance have some kind of right to hate and fear the Horde?
I'm sure there's some background lore I'm missing, but seeing as the Orc's were the aggressors in both of the first two wars, where does the Alliance get the rep that they are the "bad guys"?