I have my own theory behind the Walkers.
From what we know of Bran, the Children of the Forest are powerful magic users--the most powerful version of them are Greenseers. (One in a thousand would men might be wargs and one in a thousand wargs, greenseers). Now, in the books it's confirmed Wargs have big variances in power ranges, particularly powerful Wargs can jump into other humans, and completely inhabit their bodies. Greenseers, from what we have seen, are even more powerful completely untrained (Like Bran) than even the most powerful Warg. (Bran can inhabit Hodor before any formal training or much experience AND he doesn't need to be dying to do it.) We can also see this in Bran's initial training--unlike Warg's, he can inhabit Weirwoods and see the past with them. And as we know from legends, wierwoods are the Old Gods that first men worshiped, and they had the ability to affect things around them.
This denotes that the "Old Gods" were actually Greenseers, who affected the world through wierwoods (It's intimated they could control plant and animal life through them.)...We know the Children of the Forest had some ridiculously powerful magic, and I believe the Greenseers were the ones who actually had that magic--strong enough to shatter the land bridge from Dorn, and create the marsh.
Here's the thing--we know "blood magic" is one of the most powerful forms of magic, that death pays for life and that in death, many forms of magic get stronger (Faceless need someone's face, from death--a Warg can make a "final leap", even more powerful than his normal warging, a "permanent" leap, when he's about to die ect). What if after the truce was called, and the first men adopted the Children of the Forests ways, a greenseer was born, but because he could see into the world of men, he was corrupted. Men, because of their technology (Like bringing fire with them), were so plentiful that he could use them to produce blood magic, and increase his power.
Eventually, in secret, he used blood magic to create The Others by corrupting Children of the Forest (In one chapter it's said The Others reflective armor is like the Children of the Forests armor), and then grew strong enough to take people over when they are about to die. How he did this was through blood magic, this is why he needs Crasters babies (And why the WW raid small hamlets ect, for sacrifices)--now one other baby was sacrificed in the book, Daeny's. And when that was used to save someone from death, Drogo, he came back mindless. If the Great Other is using that kind of magic to bring people back, because they are mindless, they wouldn't be able to resist him when he used his warg magic on them--so the wights are really the Great Other's blood magic wargs. (We have some evidence that controls of the wights can be resisted, as with Coldhands--now, if coldhands is Benjen, then maybe Benjen was a warg, like Bran, and due to the magic in his blood, was able to resist the "Great Other" from taking him over.)
Anyway, the Children who resisted this corruption (This is why there are almost no Children anymore, they are The Others now) and First men fight this "Great Other" but get their asses kicked. Eventually they realize they need to use Blood Magic of their own to equal him--they literally need to fight evil with evil. And we see that in every description of AA--all of his super-magic is based around the sacrifice of others, IE Blood Magic. So maybe AA was a human Greseer, who the Children taught magic because the had none of their own left, and he used blood magic to counter the Great Other. Unlike the regular Greenseers which focused their magic around Wierwoods, the AA focused it around fire (We saw in history one of the First Men's big advantages was fire). And that's how the Red God was born. He's not "good"--he is simply a counter to the Great Other using the same kind of magic.
Just my theory.
