DnD alignments for Alliance/Horde

Alliance = Lawful Good
Horde = Chaotic Good

Or maybe switch out the Goods for Neutral?

I don't think it's as easy as saying the entire faction is one or the other. For me, my Horde character (main) would be Lawful Evil.

My Alliance Shaman is Lawful Good.

My Warlock I'd tend to say is a bit more chaotic neutral.

I think it all falls to the individual and not the faction as a whole unless you define them by their leadership.